There are varying degrees of this body posture. In fact, some are deliberate and functional, thus supporting a particular influence on a ball after release. Take special note to my selection of words - deliberate and functional. Hence there are reasons here that signify practice and perfection because of something deliberate, not causal or due to happenstance.
Then there are those body postures (moving the pitching arm across the body) that are early signs of fatigue, a momentary strain, proposition problems with a pitching surface - like mound conditions, etc. Also, you’ll find a pitcher, now and then, will simply “howitzer” a pitch with all his/her might, with very little follow though in a upright pose, thus giving the same “across the body motion”. .
In any event, this is a very good question. Not only is it topical to all pitchers, regardless of their style and pitch inventory - but, it also plays heavily on those coming off injuries, either from the post season of the prior year, or from off-season sports like football, basketball and soccer.
If you remember, Chew, I sent you a publication that I made for Rookies that made some reference to this question, but in a slightly different way. On page 31 (pictured below) you’ll see a pitcher with his delivery motion “horseshoed” at the shoulders. This finish body posture is uniquely common to those pitchers that have issues - like I described above. But, more importantly, pitchers that display this constant upper body motion that’s abnormal to their usual style, can show signs or nursing an injury - either prior to his/her appearance or during. THIS is the time to step in and start asking, “what’s up?”
The human body has a unique way of dealing with itself when confronted with an injury. Usually, the body as an unsolicited response to demand loads will actually shift those loads elsewhere. For example, let’s assume that you’re scheduled to pitch today, but last night you didn’t sleep well. You woke up and your left shoulder was stiff and not feeling like it should. Add to that, no matter how long you stand under a hot shower, get a rubdown, rotate your shoulders with the trainer, it still doesn’t feel the way it should. Your bullpen duty is a labor, forcing what you know should be your exchange of shoulders, during your “prep” and delivery - but still, the right feel is still not there 100%. Thus your glove side shoulder, being stiff, will actually be supported by more “feeling” from the right-lower side of your torso and stride leg. To the casual eye, there’s nothing to wright home about - but, a bullpen coach will have more to say about that.
As a bullpen coach - without a single word said, her/she will notice and take note. And let’s say that he/she notes that you’re about 85% for “go” and that’s OK. So, let’s say that “85%” is good for your pitch inventory and the days rest that you’ve had, so on and so forth…
But, your projected duration is expected to be only three innings at best - He/she also takes note of that and passes that on to the bench via a discreet squawk box call or note passed down with a runner.
So, as soon as the pitching coach in the dugout see’s you going to a cross body motion, repeatedly, and you’re not showing signs of being on top of your game - health wise and pitch quality wise, out you come. Then, you’re looked at by the trainers and we talk. So although going across your body isn’t a big deal for you - this might not be the case for some other pitcher in your rotation. Depends.
Good question. Go to page 31 of your book and review the generalizations that I made there. In your case, it’s important. When your glove shoulder does not rotate all the way back - allowing the pitching shoulder to complete its final part of the pitching cycle, you’ll be in line for some serious shoulder strain and lower back pain. Your delivery style with your slider can promote stress in the upper portion of your pitching arm, as well, if you show this body style to much.