OK, great, he was barreling up on the ball this year, and Parker H. thinks it's because he made a hands adjustment.
1. "Hands adjustment" is an oversimplification of what happened; it's more of an adjustment that has led to better synchronization of upper/lower half (timing).
He has demonstrated a better "stretch" at the launch position (front side and the back side are pulling in opposite directions before firing forward) where his weight/hands are now back instead of drifting forward at the ball at front foot strike.
This was something that plagued him with the Angels over the years. In his early days, he was given limited playing time and then struggled for prolonged stretches. He'd change his stance in a variety of ways over the years in efforts to find it. In 2015 he displayed an open stance with a leg kick. In 2017, after being sent to the minors to refine his swing, he came back with this hands over his head stance.
In regards to the hands adjustment, he's lowered them and kept them closer to his launch spot (rather than having to move them in sync with the pitcher). He can now just go straight back stretching instead of lowering them and then bringing them to that same spot. (This is something that helped Jorge Polanco out in 2017.) This isn't necessarily something all hitters need to do but if they are ones that get out of rhythm, it might be best to shave that distance off.
In short, Cron was a bit of a mess until last year. Part of it might be the swing refinement and part of it could be consistent playing time (without the fear of being demoted in LA).
But barrel rates don't necessarily correspond to top hitters (Joey Gallo was #1 this year in Brls/PA%; Jake Cave and Tyler Austin were in the Top 20, higher than Cron; Logan Morrison's 7.2 Brls/PA% ranked 49th, not far below Cron's 7.9%). And it wouldn't surprise me at all to see Cron's barrel rate regress to his 2017 level of 6.7% just from natural, random variation. He might fall back on some old habits without even realizing it, or he might just not enjoy quite as much good luck again.
2. Barrel rates more or less explain the amount of power potential a hitter has – not necessarily the overall hitter. Additionally, if a hitter doesn’t make a ton of contact then there overall numbers are going to be different. When you look at that top list, you can see who has some of the most raw power potential when making contact.
But Barrels have been found by the StatCast guys are being significantly important to future production. I was working on an article regarding Austin and Cave’s potential and had a correspondence with MLB’s Mike Petriello. He directed me to research by TangoTom which found that Barrels correlate to future wOBA figures similarly to BB or K rates.
More or less, this is a skill that can be consistent year-over-year but it sounds like there needs to be 250+ batted ball events before considering it sustainable (which may be flukier for Austin and Cave).
Barrels are important because over 75% of those become hits (and extra base hits) so the more BRLs/PA you have, the better your numbers (at least BABIP) should be. The rub is, the remainder of a hitter's profile, particularly in regards to making contact.
3. One of the bigger differences between Morrison and Cron is that as a right-handed hitter, Cron is not going to see the same level of defensive shifts Morrison would. Morrison saw shifts 75% of his plate appearances while Cron had a shift just 14.5% of his. As the league continues to increase the amount of shifts, Morrison's pull side open space will be squeezed out to nothing while Cron should still have ample opportunity to find vacancy.
4. I would have rather have had Josh Donaldson.