The Rocker is one of Tom House’s training drills…I wish I had video of it I could share because, like most drills it’s easier to understand when you see it than it is when you read about it.
Still, here goes:
This is usually a flat-ground drill, but there’s no reason why you couldn’t do it from a mound. It is a throwing drill so the pitcher will need a ball, glove, and a partner (usually another pitcher, who is also doing the Rocker).
The pitchers stand about regulation distance from each other. The guy with the ball comes to the set position with his normal posture. He goes into his normal delivery but stops and holds position without throwing when his stride foot has planted. Here’s what the pitcher should look like at this point:
His front foot is planted, back foot on tip-toe. Hips fully open, shoulders fully closed. Glove-side arm and throwing arm are “opposite but equal”…that is, the arms are balanced and the angles at the shoulders, elbows, and wrists are about the same for both arms.
Okay, the pitcher then rocks back into the set position and the drill starts again. Do 3 repetitions of the drill and on the final rep, don’t stop–continue through with your delivery and throw the ball to your (alert) partner.
The partner then does 3 reps, throws the ball, and so on…
This drill, like all drills, has some artificial elements to it; however, it does effectively isolate and kinesthetically train the part of a pitching delivery where the pitcher should be getting his maximum hip-shoulder separation.