Some pitching coaches want to see a high front side. For me, the height of our glove hand is a tradeoff with how easily we'll repeat our delivery: we'll create more torque with a less repeatable delivery (higher glove side), or create less torque with a more repeatable delivery (lower glove side). Obviously, there is upside to throwing harder. Obviously, there is upside to having greater control.
I like to see my guys have a firm glove hand, regardless of glove hand height. I equate the firm front side to the handle of a whip - thick and strong. While the throwing arm that follows the glove hand is similar to the thinnest, most tapered end of the whip - long and loose.
When our glove hand ends up behind our backs, our body is fighting itself and we lose energy in our kinetic chain that we could be directing into the pitch (or throw, for position players). Imagine cracking a whip that was thin throughout. There would be no thick, strong front portion to fire the back portion through.
A simple drill to work on a firm glove side: put a weighted ball in your glove while doing towel drills. The weight of the ball will force you to control it, or else it will feel very awkward when the weight of the ball carries itself behind your back.
Here's a visual of 3 of today's best pitchers with a firm glove hand out in front of their bodies at release point:
Ryan Speier is a professional pitching instructor in Tulsa, Oklahoma. He played professionally for 12 seasons, was a member of the 2007 National League Champion, Colorado Rockies, and has coached at the NCAA Division-I level. Ryan is a dad, husband, and follower of Jesus Christ. Find Ryan on Facebook here: https://www.facebook.com/SpeierBaseball/