johnny, my son is a LHP who has signed with a Pac-10 team. I'm glad you asked this question, because it is the BIGGEST detriment to your development.. "short-arming". Why? Think about it. When you pitch, and you plant with your lead foot, if you're short-arming how are you going to generate velocity? With your arm. This leads to arm problems because true velocity is NOT generated by the arm but by the trunk.
To answer your question, find a qualified pitching instructor ASAP. DON'T WAIT! Bum, Jr. worked with many instructors, but all were qualified.. his best pitching coach is a D1 pitching coach. Secondly, I suggest you concentrate on long-tossing year-round. That's what Bum, Jr. does, 5-6x week since age 11. He has NEVER taken more than 2 days without throwing. This builds arm strength, creates arm speed, and by long tossing you almost automatically stop short arming the ball.
How old are you? If you want to pitch in the future, start reading some HSBBW threads on training and conditioning. PM me and I'll give you some more tips.
If you "short-arm", you will be limited to about 83-84 MPH (tops). You could also face the possibility of long-term arm problems. If your mechanics are efficient, when your foot plants you should be in a "line". That means if someone stood directly behind you at the moment of foot plant, it would be a perfect line between the ball, your body, and your planted foot.
