The adage that power is the last tool to develop floats around every year when trying to explain why a certain prospect has or has not realized his raw power in game situations. When I first heard the idea, it made sense. A hitter’s power develops as he gets stronger getting into his early-to-mid-20s, and… that was enough for me. The problem with this concept is that many of these hitters whose power we expect to develop sometime in the future already have the ability, just not the means to use it regularly. It’s not, in other words, merely a matter of getting it done in the weight room. And oftentimes, the smooth-stroking high-average doubles hitter never gets any attention for his power, then becomes a home-run monster as he matures. As an evaluator you need to understand how that happens and when it applies to individual hitters.

For this noninclusive inquiry, I wanted to look at two hitters lumped into the first group, those believed to have the raw power to be legitimate home-run hitters and how that power has or hasn’t manifested itself in the professional game. In looking at how hitters are able or unable to tap into their raw power skills, we can have a better idea of how to evaluate whether other players will be able to develop those skills into tangible results. Xander Bogaerts and Carlos Correa provide two excellent examples of this paradigm. Bogaerts has shown he can hit for moderate power in the minors against age-advanced competition, but has not yet brought it to Boston in his young career. Correa has started to showcase his power in the early going this year, though prior to this season it was more projection than demonstration. He was touted as a five-tool prospect going into the draft, and our own Kiley McDaniel graded him out in October as having a present 60 raw power tool (65 potential) with a 55 potential game power ability, or approximately 19-22 homers per season.

If you have seen how hard these two young players can hit the ball, I doubt anyone would disagree that both of them have the strength to hit 20+ home runs, even at present. By looking at how they have swung recently and in the past, we can get an idea of what adjustments they will have to make and have already made, and whether it really is just a matter of packing on muscle, or perhaps growing more comfortable facing big-league pitching.

Correa is a particularly interesting case to me from an evaluation standpoint. He obviously oozes with tools, yet I believed there to be legitimate reason why he would not reach the 20-homer plateau for a season in his career. Fellow FanGraphs writer David Temple and I went as far as making a bet on whether Correa would ever reach 25 home runs in a year. Since I don’t bet on hitters making big changes, especially ones that have had a fair amount of success doing things naturally, I took the under. Check out a couple swings from his early minor-league career to see what gave me pause.

There is very little natural lift to this swing. Try to trace the path of his hands from start to contact; notice how the direction is down all the way to the ball, followed by a roll of the wrists to take the bat out of the hitting zone. So there is never a point in his swing here where the barrel is coming through the ball on an ascending plane. It’s all down. Of course, you can certainly still hit a ball far this way, but it is completely reliant on hitting the right spot on the ball, and doing so with a ton of force. If the direction of the swing is down, and the desired exit angle is more up, you’re not going to get all the momentum to transfer from the bat to the ball. Contrast that with this guy:

Paul Goldschmidt is an unfair comparison for any young hitter, but here I just wanted to use him as a demonstration for how the swing itself can provide opportunities to use power in game. The big differences here are evident before the hands come past the back hip.

Correa’s back shoulder is already turning in as the hips fire, bringing the hands across the middle of his right bicep, not getting into the plane of the pitch until right before contact. Goldschmidt’s right shoulder stays facing the camera, allowing the elbow and hands to drop in deeper in the zone, putting the rest of his swing in line with the desired result: a line drive or hard fly ball. Accordingly, his hands don’t roll over at contact, and his posture stays more upright on top of his legs, giving him more efficient use of his strength and the ability to lift pitches whether he catches them deep or out in front. The result: a higher likelihood of consistently generating power.

There’s obviously an encouraging trend for Correa in this department, though, so far as results are concerned (as if his fans were devoid of optimism). So far this season Correa has popped 18 home runs (eight of them in the big leagues) in about a half-season of at bats. Here are a couple of his swings from earlier this month:

If we use the same checkpoint for where his hands come through, you can see he is still bringing them down as they come forward, crossing his back arm just above the elbow. But they do level off much earlier in the zone than a few years ago. And he does still turn his back shoulder into the ball earlier than what we saw with Goldschmidt. Unsurprisingly then, he is putting up a higher-than-average rate of ground balls (48.6% to 45.4% league average), with a lot of low line drives and hard grounders. For a guy who hits the ball as hard as Correa does, it’s fun to dream on what he could do if there were more high line drives and hard fly balls.

Though there is still a lot of downward direction to his hand path, this swing is obviously much more viable for a strong hitter looking to put up good power numbers. I don’t know if I would say he’s a definite 20-homer major-leaguer yet, but this is absolutely a step in that direction. Hitters change slightly from year to year, and we have to hope this is a trend and not a natural variation. It assuredly puts Temple in a better position on our bet if the change holds or improves.

On to our other case, Xander Bogaerts has already shown he can hit for power in the mid-teens over a couple years in the minor leagues. In 2013, over the Double-A and Triple-A levels, he put up a .180 isolated slugging with 15 home runs before throwing in his first big-league dinger the same year. Last year, those numbers dipped to .123 and 12 in Boston, respectively, though it was easy to see that as a growth year being his first time at the highest level of the game. Through the All-Star break this year, those numbers have gone down even further, to a light .108 ISO with 3 homers. Obviously he’s been valuable regardless, but we’re looking specifically at power development for these two young players. So what gives?

First a few swings from his 2011 season in Single-A Greenville, where he hit 16 homers in 249 plate appearances:

Here we see some similar qualities to the earlier version of Correa. The first clip shows his hands coming across the middle of his bicep, not getting into plane until they pass his belly button, where they level off and come slightly up through contact. In the second clip, they come down to the level of the pitch earlier, but continue to go down through contact so that when they first get extended they’re pointing at the ground ten feet in front of home plate. He doesn’t roll over like we saw with Correa, but there is still a lot of “swing down” to his mechanics here. The more obvious issue is how it looks like his shoulders turn in front of hips, so much that it almost looks like he’s fooled and out in front of pitches. That leaves his upper body looking like it’s leaning toward the pitcher: not a great spot to be if your timing gets challenged consistently by better pitching.

Here he is playing for Pawtucket in 2013:

Bogaerts’ hands look better in the first portion of his hand path behind his body, and it does seem like he’s doing more work with his lower half than the previous swings. It is still apparent that the back shoulder wants to turn the hands into the zone early, so they continue to travel level or slightly down all the way to contact. He once more gets into extension pointing towards the ground, only starting to ascend again once he’s through the ball and releasing with his top hand. It is like he has to catch the ball far in front of his body to have a chance to elevate it, a couple frames after the strongest parts of his body have finished firing.

His 2014 season showed many of the same swings, though there were flashes of even better movements starting to come out, like in this home run:

And in 2015 some have looked even better:

Better balance and some obviously better lift there. BUT, the caveat with the last two is the location of the pitches. Though the swings look better when tracing his hand path and looking at his finish and balance, his first move with his hands and back shoulder is to go straight forward towards contact. Because both are relatively higher pitches and the fact that the hands start around shoulder level, a straight line to the ball ends up being a lot more on plane with the pitch than a lower location. In fact, the 2014 pitch is about half-a-foot lower, and his resulting extension is lower concurrently. This makes sense with what we have seen from Bogaerts since the start of the 2014 season, where he has only really punished pitches middle and slightly up and in. Here is his ISO broken down by location:

Because he throws his hands forward and his shoulder turn happens so early, another byproduct is the concentration of his power almost exclusively to his pull side. Over his young career, he has a .223 ISO to left field, but a .113 and .083 to center and left, respectively. There are guys who can sustain long careers only pulling the ball for power, but Bogaerts has strong enough physical tools that that should not be his only strength. Hopefully he can make the necessary adjustments to be a well-rounded power hitter. Despite his strong season thus far, like Correa he is hitting ground balls at a higher than average clip, all the way up at 51.3%. When you watch him swing at most pitches, you can see why that is:

A guy with his talent can get by as a very valuable player without being a serious home-run threat. He can join the ranks of hundreds of other hitters who had loads of power but were never fully able to harness it.

Without going into too much discussion, I wanted to show a few clips of another hitter whose power arrived late, in a similar way to how we hope these two young players will develop: Carlos Gomez. Here he is in 2010 and 2011, throwing his hands at the ball:

And here he is in present day doing what he’s now known for:

Notice anything different about how his hands work?

There are a few interesting points illuminated by how both Bogaerts’ and Correa’s power has developed. Hitting for power in batting practice, or even the minor leagues, does not guarantee being able to showcase it at the big-league level. Also, when projecting a player as he matures, it is irresponsible to believe he will develop game power simply by getting stronger. If Bogaerts in particular doesn’t change something about his swing or mental approach, he may not be more than a mid-teens pop kind of guy. Mechanical and mindset changes don’t just happen naturally, and we cannot assume that the catch-all “great makeup” label with which every top prospect gets branded on their scouting reports is enough to ensure they will figure things out. Carlos Gomez, like many talented young hitters who hit the ball hard, had to consciously change his approach and swing to fully utilize his physical gifts. So will these two.