What is it? Pictures would be nice
something that is natural and you allready do. It’s when the shoulder blades get pinched and the chest thrusted out trust me you will mess up your mechanics if you focus on this because you already do it naturaly when pitching or throwing.
So just don’t worry about it.
It’s pretty natural, but it’s good to hear about how to do it properly. It’s just basically ‘pinching of shoulder blades’
Well the only way to do it properly is to ignore doing it, because it occurs during natural throwing motion.