This is part two of a four-part series by Charlie O'Connor describing and evaluating the penalty kill of the Philadelphia Flyers during the 2015-16 season. Yesterday, we outlined and defined the different formations that the Flyers' penalty kill used in their forechecking and defensive zone strategies. Today, we've delve into the manually-tracked forechecking metrics.

Yesterday's breakdown of the specific formations used by the Philadelphia Flyers on the penalty kill was, in many ways, meant to be an introduction to this entire series. In order to delve into tactics, it was necessary to make sure that every reader possessed a base level understanding of penalty kill X's and O's, which hopefully I was able to provide.

But the real core of this series is a deep dive into the statistics behind those formations, all of which were manually tracked by myself. Today, the focus is on the forechecks. First, we'll evaluate which forechecking schemes were used most by the Flyers during the entirety of the 2015-16 season. Then, we'll look at their usage over time, to check if the coaching staff made tactical adjustments throughout the year. We'll close out our analysis of the Flyers' forechecks by using metrics to evaluate which types were most and least successful in helping Philadelphia to execute an effective penalty kill.

Which forechecking schemes did the Flyers use the most in 2015-16?

We previously defined four main forechecking formations -- the Tandem Pressure, Same-Side Press, Passive 1-3, and Retreating Box. I was able to track 80 of the Flyers' 82 regular season games during the 2015-16 season (January 7th and January 13th were unavailable on NHL.tv), noting each time a specific formation was used by the team during a 4-on-5 penalty kill situation.

The Flyers primarily utilized the Passive 1-3 as their base formation. As I noted yesterday, their version of the Passive 1-3 gives the second forward (F2) extra freedom to "step up" in the middle of the ice, but it still fits the regular parameters of the forecheck.

As noted yesterday as well, the Passive 1-3 and the Same-Side Press are very similar in their initial formations. If you look at the former as the "safe" version and the latter as the "aggressive" version of the same general forechecking concept, then it becomes fair to say that the Flyers were clearly a base 1-3 penalty killing team in 2015-16. The Passive 1-3 and the Same-Side Press were used a combined 56.63 percent of the time, a solid majority over any other tactic.

The Retreating Box and the Tandem Pressure were Philadelphia's secondary tactics, with the former coming in at 24.57% of all forechecking situations and the latter at 11.04%. The "Other" category primarily consisted of situations such as slow line changes and quick entries off neutral zone faceoffs, when a proper neutral zone forecheck could not be executed.

Did the Flyers' forecheck usage change over the course of the season?

We've now established how often each of the four main forechecks were used during the entirety of the 2015-16 season. But what the full-year percentages don't tell us is whether tactics changed in the neutral zone as the year progressed.

To answer that question, we'll look at the usage percentages by month to see if there were any significant spikes or declines in the the four individual forechecks. One look at the below chart answers that question quickly.

The predominant usage of the Passive 1-3 stays relatively stable throughout the season, ranging from the low-30% region in February to north of 50% in January. Regardless, the Passive 1-3 was always first or second in the rankings. It's the dramatic increase in usage of the Retreating Box forecheck that is the real eye-opener. Corresponding with a drop in dependency on the Same-Side Press and Tandem Pressure, the Retreating Box formation was a real rival to the Passive 1-3 over the final three months of the season.

The aggressive Tandem Pressure's high point came in October, and only to be used less and less as the year progressed. On the other hand, the Same-Side Press seemed to be a consistent tool in Philadelphia's arsenal until February, when it became a second thought beside the Passive 1-3 and Retreating Box. When the Flyers used the 1-3 formation late in the year, they generally backed up through the neutral zone rather than let their forwards attack.

Which forechecks were most effective for the Flyers?

Before we jump into this section, let's first ask an obvious question: how can we measure the effectiveness of a forecheck? Or in more basic terms, what should an effective forecheck be trying to accomplish?

To start, let's look at possible outcomes. Just like at even strength, there are three potential results when an opponent tries moving the puck through the neutral zone against a forecheck. The first is that they successfully navigate the middle of the ice and carry the puck into the offensive zone with control. The second sees them enter the offensive zone, but are forced to relinquish possession (or dump the puck) in order to get there. The third and best possible outcome for the defense is that the rush is broken up entirely, and no offensive zone entry occurs.

At even strength, it's been repeatedly proven that controlled entries (carry-ins) generate more shot attempts on average than uncontrolled entries (dump-ins). But does that hold for power play situations as well? Luckily, Corey Sznajder tracked every 5-on-4 power play entry for the 2013-14 season, giving us a decent sample size to determine the league average outcome for PP controlled and uncontrolled entries.

Outcome 5v4 Power Play Even Strength (5v5) Controlled Entry 0.66 unblocked shot attempts 0.66 unblocked shot attempts Uncontrolled Entry 0.41 unblocked shot attempts 0.29 unblocked shot attempts

Just like at even strength, controlled entries are more valuable than uncontrolled ones, though the gap does shrink on special teams. This make sense -- puck retrievals on power play dump-ins should be easier to execute against an opponent with a man in the penalty box over an opponent at full strength.

A full breakup of the rush is obviously the best case scenario for the penalty kill. But armed with this data, we can be confident that a neutral zone forecheck capable of forcing a high percentage of dump-ins is also an effective one.

A second method we can use to judge penalty kill efficiency comes from the great work of Arik Parnass this past season via his Special Teams Project. Parnass was able to determine that the best current way to measure the efficiency of power plays was to evaluate them based upon their ability to quickly enter the offensive zone and set up to prepare for a dangerous attack.

His new statistic, ZEFR Rate, measured the percentage of zone entry attempts that either resulted in a scoring chance off the rush or a successful offensive zone formation by the power play. Most importantly, Parnass found that it was the most predictive measure yet in determining future power play scoring efficiency.

We can also use ZEFR to judge Philadelphia's penalty kill. If it is the best stat yet in determining future power play effectiveness, it most likely will serve the same purpose in helping us to judge a team's penalty kill prowess. To that end, I tracked the ZEFR rate for Flyers' opponents against each of the four main forechecks, in addition to each forecheck's percentages for breakups created, and controlled/uncontrolled entries allowed. The final metrics are below.

Forecheck Type Breakup Percentage Dump-In Percentage Carry-In Percentage ZEFR Against Percentage Tandem Pressure 39.29% 17.86% 42.86% 38.10% Retreating Box 28.34% 34.76% 36.90% 44.39% Passive 1-3 23.00% 27.00% 50.00% 46.00% Same-Side Press 24.43% 37.40% 38.17% 48.85%

We'll do a deep dive into the implications of these percentages in a second, but let's start with some high-level analysis. To start, the Tandem Pressure was both the most effective forecheck in forcing full breakups, and had the lowest ZEFR Against Rate, meaning that opposing power plays only were able to successfully set up in the offensive zone (or generate a rush scoring chance) 38.1% of the times it was employed by the Flyers.

Retreating Box comes in second in both Breakup Percentage and ZEFR Against Percentage, and it was actually the most stingy in preventing carry-ins. The variations of the 1-3 come in third and fourth, as the Same-Side Press did a better job of preventing controlled entries while the Passive 1-3 posted a more efficient ZEFR rate, implying that forecheck was more successful in thwarting structured formations from the zone entries it allowed.

What does this data teach us about the forechecks?

Armed with these metrics, we can now develop more informed theories on the strengths and weaknesses of each forecheck than we were able to do in Part One of the series.

The Tandem Pressure grades out just as it looks on tape -- a high-risk, high-reward forecheck. Its Breakup Percentage blows the competition out of the water, usually due to the cycling forwards forcing a turnover deep in enemy territory. But if those forwards fail in their mission, the ensuing rush will almost certainly be of the controlled variety. Still, this data implies that the risk might be worth the reward, as not even a 42.86% Controlled Entry Against Percentage could hold back the Tandem Pressure from achieving the best ZEFR rate among our forechecks.

Retreating Box comes out as the most balanced forecheck here. It breaks up a fair amount of entry attempts (second highest Breakup Percentage) while also allowing relatively few carry-ins. As was noted in Part One, this forecheck functions as a middle ground between the safety of the Passive 1-3 and the aggressiveness of the Same-Side Press, and the numbers bear that out.

But it's the comparison between the Passive 1-3 and the Same-Side Press that proves most intriguing. Looking at their respective Dump-In and Carry-In Percentages, it seems to be a sure thing that the Same-Side Press would be the more effective forecheck by ZEFR. Instead, the Passive 1-3 wins out. Why?

One theory is that the Passive 1-3 -- while weak in the neutral zone -- is at its most effective structurally in preventing power plays from setting up once in the offensive zone. After all, the forecheck puts three penalty killers right across the team's blue line, cutting down on the distance necessary to travel for any of the three to meet the puck carrier post-entry.

The Same-Side Press, on the other hand, expends its most deadly bullets in the neutral zone. If the puck carrier can make it past F2, who is charging forward rather than retreating, then the power play only faces two penalty killers in the first few seconds of their zone entry. In theory, that makes it much easier to set up in formation than in a scenario facing three penalty killers.

Data lends credence to the theory. Now, we'll not only look at overall ZEFR Against Rate, but also ZEFR Against Rate on entries. Essentially, this new stat measures what percentage of successful entries by the opposition actually turned into full power play formations.

Forecheck Type Overall ZEFR Rate Against ZEFR Rate Against on Successful Opponent Entries Passive 1-3 46.00% 59.74% Retreating Box 44.39% 61.94% Same-Side Press 48.85% 64.65%

Here we see where the Same-Side Press suffers the most. While the Passive 1-3 allows just 59.74% of its successful entries against to pass the ZEFR qualifications, the Same-Side Press is almost five percentage points higher, at 64.65%. That weakness nullifies any advantage gained from forcing more dump-ins, because those dump-ins are more likely to be retrieved against a Same-Side Press versus a Passive 1-3.

Obviously, there's only so much stock we can put in data from one NHL team. It's certainly not enough to advocate for one forecheck to be intrinsically superior to all others, or one to be fatally flawed. But it does give us new theories to ponder on a league-level, and for the Flyers, a road map to making their current penalty kill more efficient in the neutral zone. On Thursday, I'll return to this data when I provide my overall recommendations for optimizing the penalty kill in Part Four. Tomorrow, however, I'll break down the data from the other aspect of a penalty kill -- defensive zone play.