This is part one of a four-part series by Charlie O'Connor describing and evaluating the penalty kill of the Philadelphia Flyers during the 2015-16 season. Today, an introduction to the PK systems of the Flyers.

Entering their first-round playoff matchup against the powerhouse Washington Capitals, the Philadelphia Flyers had legitimate reason to be optimistic they could spring the upset. The Capitals were formidable, but the Flyers actually had played like the superior team over the season's final 25 games.

During the first nine periods of the series, Philadelphia hung right with the Caps at even strength. They outshot them, forced the Caps to play dump-and-chase hockey, and broke even from a scoring chance standpoint. But they found themselves in a three-games-to-none hole due to a complete inability to slow down Washington's power play.

Penalty killing was an issue for the team all year long. In fact, 2015-16 was the second straight season the Flyers ranked in the bottom-half of the shorthanded efficiency charts, a far cry from the preceding two years when they ranked in the league's top-ten. A formerly stingy PK was now a weak point, and the playoffs only shone a spotlight on an already-troubling trend.

But a comprehensive evaluation of a penalty kill is a notoriously difficult task. Teams usually spend between 400 and 500 minutes of a season shorthanded, compared to over 4,000 minutes at 5-on-5. This makes single-season PK data fairly noisy -- in terms of sample size, it's equivalent to only about 10 regular season hockey games. A penalty kill could be fine structurally, but hamstrung by poor goaltending performances or unlucky bounces. At 5-on-5, inflated (or deflated) percentages have more time to return to proper balances. Penalty kill metrics, on the other hand, are particularly volatile.

Still, there are ways to thoroughly evaluate special teams. Analyst Arik Parnass did this last year with his Special Teams Project, combining extensive tape study with manual tracking to measure performance.

I'll use similar tactics in order to better understand the penalty kill of the Philadelphia Flyers. First, I'll present the team's preferred strategies, both in the neutral and defensive zones. Next, I'll measure which tactics were used most often, and which were most successful from an entry and shot prevention standpoint. Finally, we'll use our new knowledge to evaluate the penalty kill as a whole, from what is "wrong" with the unit to adjustments that could improve it.

But in order to understand why the penalty kill has struggled so much, it's necessary to break down exactly what is going on from a tactical standpoint.

That's why Part One of this series has no numbers or stats. Instead, it's Penalty Killing 101 with a Flyers slant. Today, I'll detail the Flyers' most frequently used neutral zone forechecking strategies, and also their general defensive zone philosophy. The bulk of these concepts come from Hockey Plays and Strategies by Walter and Johnston -- an invaluable resource for those looking to better understand the X's and O's of hockey.

My breakdowns are incomplete descriptions of the Flyers' tactics, as each formation likely comes with specific "If X happens, then do Y" directions from the coaching staff. But hopefully the diagrams and videos help to provide a baseline for the remainder of this series, allowing us to use our newfound tactical knowledge to evaluate which strategies were most effective for Philadelphia last season.

Forechecking on the penalty kill

Forechecking is a term tossed around often during broadcasts, but it usually describes a team's efforts to recover the puck while in the offensive zone. That is an obvious example of forechecking, but it's not the only type.

Neutral zone forechecks have the same goal -- pressure puck carriers with the intent of forcing them to give up possession -- but per the name, they tend to occur in the middle of the ice. The Devils' famed "Neutral Zone Trap" that took the league by storm in the '90s is probably the best-known example of a neutral zone forecheck.

Penalty kills don't provide many opportunities for aggressive offensive zone forechecks, since the primary aim isn't to create offense. It's to stifle attempts to generate shots, quality chances, and specifically goals.

But before an opponent can fire away at the net, they need to move the puck up ice in the first place. This is where a neutral zone forecheck comes into play. Every penalty kill has set formations they utilize to either halt an opponent's rush in the middle of the ice, or at least force them to dump the puck into the offensive zone.

During the 2015-16 season, the Flyers primarily used four distinct penalty kill forechecking strategies:

Tandem Pressure

Same-Side Pressure

(Passive) 1-3

Retreating Box

Tandem Pressure

We'll start with the most aggressive forechecking scheme that the Flyers employed. It's called the Tandem Pressure, and philosophically, it's intended to put maximum pressure on a power play while they are trying to exit their zone.

The bulk of the work is done by the two forwards, the eponymous "tandem." The first forward (F1) initiates the pressure by directly challenging the opposing puck carrier. After providing that pressure, F1 circles back towards the neutral zone, but is immediately replaced by the second forward (F2), who now takes up the attack position. The two forwards switch spots for as long as the forecheck is in effect, with the goal of preventing the opposition from ever exiting their own zone cleanly.

First, we'll look at a static diagram, and then an example of the Flyers' version of the Tandem Pressure in action.

The defensemen have little to do with the forecheck -- they hang back in the neutral zone and let the forwards do the heavy lifting. The key to this strategy's success is the forwards constantly switching and working in tandem. If they seamlessly trade off their assignments, it creates the perception for the opponent of being under constant duress, even if the pressure always comes one-player-at-a-time.

A perfect example came in Philadelphia's November 5th game against the Calgary Flames.

Claude Giroux initiates the press, disrupting a handoff behind the net between Johnny Gaudreau and Mark Giordano. But as Giroux circles the net and begins to skate back up ice, Chris VandeVelde moves in. He attacks the new puck carrier, forcing Calgary to again reverse the puck around the net. VandeVelde chases, attacks the recipient of the pass, and turns back up ice -- only to replaced by Giroux yet again, who now puts pressure on the new puck carrier. Giroux and VandeVelde "trade off" no less than four times, burning twenty seconds off the Flames' power play without ever letting them exit the zone.

When the Tandem Pressure is working, it's a thing of beauty. But its strength -- the aggressiveness -- is also its greatest weakness. If the deepest forward fails to disrupt the puck carrier, the PP unit could easily trap him behind the rush. Slowing down five oncoming attackers in the neutral zone with only four players is hard enough for a PK. It's close to impossible with just three. And if a Tandem Pressure goes completely wrong and both forwards get trapped, five attackers blast into the neutral zone against two terrified defensemen.

High-risk, high-reward. That's the Tandem Pressure in a nutshell.

Same-Side Press

The next two forechecking strategies are linked for multiple reasons, not least of which because the initial formations look nearly identical. We'll start with the more aggressive of the two -- the Same-Side Press.

In this forecheck, the penalty kill lines up in a 1-3 formation, with the first forward (F1) significantly ahead of the other three members of the unit, who are positioned in a horizontal line across the ice. The second forward (F2) is stationed in the center of the line behind F1, flanked by both defensemen. F2 usually starts out a bit ahead of the defensemen, for reasons that will be explained soon.

Here is a diagram of the initial look of the Same-Side Press, to illustrate the positioning of the players.

Just as in the Tandem Pressure, the bulk of the work in the Same-Side Press is done by the forwards. But here, the goal is for F1 to bait the opposition into moving the puck up a specific side of the ice. This lets F2 quickly close in and disrupt the rush, either tying up the puck carrier along the boards or forcing a dump-in. The defensemen skate backwards with the rush, in position to chase the puck if it is dumped into the zone as a result of the press. They also prepare to defend against passes in the neutral zone that try to jump past the press.

Here is a diagram of the Same-Side Press in action.

If this reminds you of a certain neutral zone strategy used at 5-on-5 intended to "trap" opposing forwards along the boards and clog up the middle of the ice, pat yourself on the back. The Same-Side Press, in many ways, is the closest that a team can come on the PK to replicating a neutral zone trap in the classic sense.

Of course, the lack of a fifth forechecker in the formation opens up extra passing lanes that wouldn't exist at even strength. That's why D2 must read the play and watch for a pass to his side before choosing to retreat in anticipation of a dump-in.

Now, let's take a look at the forecheck in a game.

Calvin De Haan starts from behind the net. Immediately, Sean Couturier (F1) angles De Haan to move up the left side, as he is blocking the right side skating path and the likely passing lane. As the defenseman skates up ice, Couturier continues to direct him to his left, protecting the right lane.

By the time De Haan passes his own blue line, he's already fallen into the Flyers' trap. Matt Read (the F2) meets De Haan just as he passes the red line. The puck carrier's only option to avoid a collision or a turnover is to dump the puck in, sacrificing possession. Mark Streit (D1 in this situation) even has a stride or two on De Haan as he attempts to retrieve the puck.

The strength of the Same-Side Press is its combination of structure and aggressiveness, and it provides a clear road map to neutral zone breakups and forced dump-ins. The forecheck also has two key weaknesses -- one structural, and one of personnel requirements. The structural issue is that once both forwards commit to the same side on the press, that leaves the entire other side wide-open for a trailer. Sure, D2 is there, but an accurate pass between the two forwards can result in space for a trailer to build up speed uncontested in the neutral zone.

Notice how Sean Couturier anticipates this potential issue in the above video. As he directs De Haan to the preferred side, he briefly uses his stick to block the lane of a potential drop pass, realizing before De Haan that is the only real way to "beat" the press at that point in the play.

This brings us to the other limitation of the Same-Side Press -- it needs forwards with plus acceleration, anticipation and defensive instincts, such as Couturier. If F1 can't anticipate the desired route of the onrushing puck carrier, the press fails because he won't direct him to a specific side of the ice. And if F1 does his job but F2 is late recognizing the forecheck or lacks the acceleration to cut off the forward, the press fails because the puck carrier was never truly contested.

A team needs good personnel up front to master the Same-Side Press. If personnel is lacking, the team may resort to our next option.

Passive 1-3

The Passive 1-3 forecheck is the less ornery cousin of the Same-Side Press. At first glance the formations look almost identical, except for the fact that F2 often positions himself right in line with the two defensemen rather than a bit ahead.

F1 again puts a degree of pressure on the puck carrier. But in this forecheck, F2 does not race forward to cut him off. Instead, F2 sags back, along with the two defensemen, holding tight at the blue line.

The telltale sign of a Passive 1-3 versus a Same-Side Press is the actions of F2. Generally speaking, if he aggressively moves forward in the direction of the puck carrier, it's a variation of the Same-Side Press. If he skates backwards or remains in the center lane waiting for the play to come to him, it's likely a Passive 1-3.

The lack of activity from F2 places the onus on either F1 to disrupt the rush, or the defensemen to stand up the puck carrier at the blue line and force a turnover or a dump-in. Here is an example of the latter.

Pierre-Edouard Bellemare harasses P.K. Subban as he moves the puck up ice, but Chris VandeVelde holds in his center lane and retreats in line with his defensemen. The trio begins to challenge right around the blue line, and Mark Streit is able to force Montreal to relinquish control of the puck just inside the Flyers' zone.

The Passive 1-3 and the Same-Side Press aren't just close relatives because of similar formations. A failed Same-Side Press can often look indistinguishable from a Passive 1-3 on tape. After all, if F1 fails to angle the puck carrier in the desired direction, it makes sense for F2 to hold the center lane rather than chase the puck in a futile effort. Also, if F2 is late in his reaction to a successful Same-Side Press, he might choose to hold his spot as well. This link between an unsuccessful Same-Side Press and the Passive 1-3 will become important in Part Two of this series, when I evaluate the efficiency of the forechecks themselves.

Finally, it needs to be clarified that the Flyers generally run an aggressive variation of the Passive 1-3. Many teams task the F2 with primarily aiding in puck retrieval in this formation. Philadelphia, however, gives the F2 a bit more freedom to "step up" in the middle of the neutral zone and attempt to break up the rush, even if he cannot execute a true Same-Side Press.

Here, Umberger basically lets Brandon Davidson skate the puck up the center of the ice uncontested. In some versions of the Passive 1-3, this would be disastrous, as Davidson will be able to pick up a great deal of speed as the back-three penalty killers retreat. But in this case, Matt Read (while still retreating) actually begins to challenge Davidson as he reaches the red line, forcing a dump-in. It's not really a Same-Side Press, though you could argue that Umberger is directing Davidson to the center of the ice in a sense. Really, it's more of a Passive 1-3 with extra freedom granted to F2. The Flyers used this tactic regularly in 2015-16.

Even the more aggressive version of the Passive 1-3 has an obvious weakness, right in the title. It is, by nature, a "passive" forecheck -- first and foremost meant to prevent disaster. Rarely will you see a stretch pass get behind a competent Passive 1-3, but you will see a fair amount of controlled entries into the offensive zone unless F1 is very effective at creating pressure. If F1 can't slow down the rush, the puck carrier often doesn't hit the next wave of forecheckers until close to the blue line, which gives him time to build up speed or find an even faster open man.

Retreating Box

We'll close out our look at the Flyers' main forechecking schemes with the Retreating Box. Unlike the Same-Side Press and Passive 1-3, it's immediately obvious from a structural standpoint when a team is using the Retreating Box formation. A change of pace from a 1-3 formation, the Retreating Box is more of a 2-2.

The key to understanding the Retreating Box from a structural standpoint is to think of it as a literal rectangle retreating down the ice. The horizontal invisible line made by F1 and F2 stays parallel with the line of D1 and D2, and the same goes for the vertical lines of F1/D1 and F2/D2.

However, this only holds until the puck gets in range of the front of the box (F1 and F2), which usually occurs around the red line, kicking off the pressure portion of the forecheck. Remember how the Same-Side Press baits the puck carrier into going up the sides before trapping him? The Retreating Box has a similar concept, daring an opposing power play to use the center before surrounding the puck carrier once he commits.

If the power play forgoes the "open" center and tries to move the puck up the side of the ice instead, the closest penalty killer to the puck has very little ground he must cover between his original positioning and the boards, making it possible for a quick challenge.

Unlike the Tandem Pressure and Same-Side Press, which can be categorized as more "active" forechecks where the penalty kill dictates the decision-making of the PP, the Retreating Box (while not passive) is a "reactive" forecheck. It waits for the power play to reveal its strategy before the closest member of the forecheck attacks. And since the ideal Retreating Box is a square with all of the invisible sides equal, it's difficult for a power play to find an obvious positional vulnerability.

A weakness of the Retreating Box, however, is that it can be exploited by power plays that change direction quickly. Once the power play enters the box's range, at least one player usually commits to attack it, leaving him out of position if the puck is quickly passed in another direction. Drop passes in particular can flummox the Retreating Box, forcing the remaining penalty killers to drop into an improvised Passive 1-3 to limit the damage.

Still, the Retreating Box serves as a strong middle ground forecheck, between the structure and passiveness of the 1-3 and the attacking Same-Side Press.

Defensive Zone Formations - Triangle +1 and the Czech Press

Compared to the advanced algebra of neutral zone forechecks, defensive zone formations are like basic arithmetic. There's only so many tactics a four-man unit can employ when facing five of the most skilled players on their opponent's roster.

Still, defensive zone play on the penalty kill remains worth analyzing. As I explained in April, it was a complete absence of pressure in that area from Philadelphia's PK that helped allow the Capitals' power play to run roughshod over the Flyers, and only after adding more aggressive tactics did they begin to slow it down. It's more that defensive zone penalty kill strategies can mostly be placed into two categories -- passive and attacking -- and there aren't many variations of either.

For the most part, Philadelphia leans towards the former strategy. Specifically, they almost exclusively employ a Triangle Plus-One defensive zone formation while on the PK.

The general concept of the Triangle +1 is that three penalty killers form a triangle shape down low, protecting the slot and net. The fourth penalty killer is the "plus-one," focused on getting in shooting lanes, angling the play in certain directions, or pressuring the puck carrier. The key to understanding the Triangle +1 is knowing that the player functioning as the "plus-one" can shift mid-play, depending upon the location of the puck. If the play quickly transitions from left to right, the high forward can sag back into the triangle while the lower forward takes over the "plus-one" role.

Here's an example of a continuously-shifting Triangle +1 formation against the Canadiens.

Throughout this clip, VandeVelde and White swap roles as the "plus-one" forward as Montreal moves the puck around the horn. Every time either forward relinquishes his role as the high man, pay attention to his positioning. He invariably sags back into a spot where you can easily trace a triangle of him and the two defensemen in the mostly-stationary formation, while the "plus-one" forward roams more freely.

One thing you may notice while watching the above clip is that regardless of which forward was the "plus-one," there isn't much direct pressure placed upon the puck carrier. The threat of pressure is there, but there is no actual direct attacking from the high forward in the PK formation. This was a season-long trend, and why I refer to Philadelphia's base PK defensive zone formation as a "Passive Triangle +1."

But there is a version of the Triangle +1 that advocates more pressure from the high forward. This has been dubbed the "Czech Press," and it's the adjustment that Philadelphia made to its PK in the latter half of their series against the Caps in order to stop the bleeding.

The same concepts of the Triangle +1 apply, but in the Czech Press, the "plus-one" forward is very aggressive. He actively attacks the high man at the blueline (usually a defenseman), and in the event of a pass to a forward on the half boards, he "presses" down on the new puck carrier to cut off space and time.

In this clip, Giroux initiates the pressure, first attacking John Carlson up top, and then quickly moving to push Nicklas Backstrom down below his own blue line. When Backstrom moves the puck back up top, Giroux settles into the triangle, while VandeVelde replaces Giroux as the "plus-one" forward and continues the press.

When the Flyers did use pressure-based tactics in their defensive zone penalty killing, the Czech Press was their usual choice. But Philadelphia mostly played a Passive Triangle +1, with just enough Czech Press sprinkled in to keep power plays from removing it from their minds.

As with the neutral zone forechecking options, the decision to use passive or aggressive tactics in the defensive zone comes down to a risk/reward evaluation. The Passive Triangle +1 focuses on blocking shots and keeping an opposing power play out of the most dangerous areas. It hopes the power play will make a mistake, whether that be a fumble of the puck, a bad pass, or a shot that either misses the net or is blocked.

The Czech Press, on the other hand, looks to actively force opposing power plays into mistakes. The result is more turnovers, but also more opportunities for dangerous chances, due to the chaotic play of the "plus-one" forward. Rapid puck movement has the aggressive forward constantly chasing, and can turn the formation into a "Triangle Plus-Nothing," with three penalty killers in the slot just waiting to be eaten alive.

In tomorrow's segment, we'll evaluate which neutral zone strategies were used most frequently by the Flyers in 2015-16, and which ones were most effective.

Special thanks to Prashanth Iyer for his help on this article.