A caveat: I was speaking from the standpoint just of taking criticism/instruction. This was mainly focused around the fact that I played A LOT of timpani in high school and college.
I had always been brought up on the traditional matched grip, with the arms at a "^" angle and the palms down, stick between thumb and forefinger with the others curled around as "guides" if you will.
Some call it the "German" grip, too.
He was trying to get me to roll my wrists over 90-degrees, so that my palms were facing inward/facing each other, with my thumb always on top. He also wanted me to bring my arms in a bit to my sides.
Some call it a "French" grip.
It doesn't seem like much when you think about it; shoot, you're just turning your wrist. But, when you've been taught to play a certain way for 10 years in every lesson you've taken, it's hard to change. You really have to be willing to WANT to work on it. But, it did force me to really strengthen and use my wrists more, since you can't really sustain any kind of rolling or rhythm with your elbows when your palms/wrists are turned over like that.
My point was that I didn't really want to listen to the guy, because I was comfortable with my old grip and didn't see the point. But, when I really dropped my stubbornness and realized there was something to be learned from this guy, I was able to take things from it, especially in my mallet-playing. And, really, my set playing now is kind of a hybrid, with my right hand playing a lot of the "French" grip style on the hats/ride, and my left playing the more matched/"German" style on the snare.