There is no dispute that players have different usage of their hands and arms.
The point here is you are teaching it as a "way to hit". A way to improve. Do this and you'll be better.
That is far from the truth.
The reason why a Sheffield or a Bonds or a any other "army" hitters do that is they because they can. They can generate better batspeed by doing so but that doesn't mean you or I can.
The difference is in their center. They already understand how to load/unload their center. They are experts at it. They have perfected it. Their quickness is maxed out. So, they then turn to improving batspeed. Quickness allows you to catch up to all the great pitching. Batspeed adds distance to their hits. They can do that. They are rock solid in their center. They can experiment with adding speed.
The problem is, you and I can't. And neither can 95% or more of the players we encounter. They don't have their center under control. They don't understand the load/unload process. They are not as quick as they need to be.
So, you come along and say "do this with your arms". At their current level of competition and quickness, they have success. They look at you like "wow, that really made a difference." Yet, 2, 3, 5 years down the road in no longer works. The pitching is much faster. The quality of the breaking pitches are much better. The reaction time is greatly diminished. And your students generate really good batspeed but can't get the barrel to the ball because it takes too long for them to launch.
Unless and until they learn connected rotation from their center their "arm action" style of hitting will take them as far as their athletic ability goes. As long as they are "athletically gifted" as much or more than the competition they will be fine.
But, to advance a far as they possibly can, they will need to add "skill" to their athletic ability.
This is why so many "obvious" D-1 stars don't make it very far in professional baseball. They have never added "skill" to their "athletic ability".
This is also why someone like John Olerud, who is known to have the one of the slowest batspeeds in professional baseball, has the success he had. I've heard his batspeed in considerably less than most mlb players. But his "skill" level is very high.
Finally, a good coach is one who knows the difference and won't let the players read his press clippings. The coach who tells a player what he needs to hear versus what he wants to hear is extremely valuable down the road. The problem is all the misinformation and therefore the lack of quality coaching at all levels of baseball.
In this country it is ridiculous how small a percentage of kids that reach the top level of the game. They have been very misguided. In fact, whether one makes it or not is purely chance or "luck of the draw". They "discover" what works purely by chance through their own trial and error. Nothing wrong with trial and error. That's how everyone learns. But, trial and error in this country is set back thousands of hours by things like "arm action" hitting philosophies.
I'm still waiting for video of the hitter.