importance of staying closed

cchs good to hear! One way to illustrate the importance of staying closed is to do both: 1) START in the foot-plant position with the glove, the body and the ball (behind you) in a perfect line (closed), then 2) START in the foot-plant position with the ball/hand flown open (maybe 12-18 inches in front of an imaginary straight line extending out behind you from the glove and body.

Now, from these positions, fire the ball to the catcher. How hard can you throw in position "1"? Position "2"?

You'll find MUCH harder from position 1. Flying open usually breaks the kinetic chain needed for velocity.

What is the exact definition for "flying open"

does anyone know?

My understanding is that flying open is when your front shoulder begins to square towards the plate before your throwing arm has a chance to catch up to it. This is known as dragging the arm. This will put excessive stress on the shoulder and usually cause the pitch to either wind up high and inside or low and outside. The reason for that is the pitcher is either releasing too early because the arm is lagging behind the body or the pitcher whips his arm through trying to catch up to the body.

Ideally you want the arm to be in synch with the upper body (shoulders). It should be a straight line from shoulder to shoulder and through to the pitching elbow. This way you are utilizing the strenght of the core muscles as well as the arm to generate velocity. It is also much easier on the shoulder.

Hope this helps.

The shoulders can "fly open" which means they rotate earlier than they should.

The glove arm can "fly open" which would very likely lead to the shoulders flying open.

The front foot/leg could "fly open" into foot plant which would probably lead to early hip and shoulder rotation.