...what your arm/chest look like when you throw really hard?
This guy looks like he's throwing a disc.
The first pic that came to mind was me at CHC08 getting humped by the mark. Post-release, rather than pre-, but the similarities incl. off-hand positioning (I do get called for pushoff fouls from time to time. I believe these are more accurately described as "upper-body rotation fouls".), and... not sure what else.
That gallery, however, included the following pics: 1, 2, 3, 4, 5, 6, 7.
Things to look at: Off-arm/hand positioning, positioning of throwing arm relative to shoulder, elbow angle(& valgus stress!), facial expression, hips, and more.
Back to the original photo. Consider the engagement of muscles from all of the pectoralis major (clavicular [aka "upper", aka "attaches to clavicle"], sternal [aka "middle", aka "attaches to sternum"], and costal [aka "lower", aka "attaches to the ribs"] sections, to everything in the general frontal/side-al ab region, to the ever-sneaky coracobrachialis. Consider the engagement of the muscles on the opposite side of the body. Consider that the motion is essentially a full-body twist/rotation with the spine/sternum as the pivot.
Consider the elevation of the handballer's arm. Consider the extension of the muscles at various arm positions (compare to the frisbee photos). Consider the paths that they draw to the sternum.
Try an activity! Put your non-throwing arm (L for Righties) on your throwing chest. Now, pin your elbow to your side and externally rotate. If this confuses you, put a disc in your throwing arm, and slowly replicate a throwing motion with your elbow pinned to your side. Feel how the muscles in your chest move. Note how far back you can move the disc. Now, straighten your throwing arm at shoulder height. Rotate similarly with elbow locked. That is, take your arm back as far as possible, and then swing it forward to point straight ahead. Note the difference in distance behind your back. Notice the difference in what your non-throwing hand feels in your chest muscles. Now, reach UP and BACK with your straight throwing arm. Swing it forward so that it ends at chest height in front of you. Notice any differences. Finally, reach UP and BACK with your throwing arm, but when you reach the end of the line, bend your elbow to ~115 degrees. Swing your arm forward to the same end point. Note the differences.
Which of these feels strongest? Which, if you provide resistance, is the strongest? Which engages more muscles? Which engages more muscles in a straight-line force-production rather than bending through your shoulder joint?
Back to the handballer. That dude is about to rip the shit out of the ball. He's calm, he's got his eyes on the prize, he's got good form, he's taking the ball high rather than low (see: tennis.), he's engages the muscles from his wrist through his arm through his shoulder through his abs through his hips. I bet if he had no pants, we'd see good activation in the hips/legs as well.
But what about that awkward off-hand? What IS it doing? Why is it so similar to what frisbee players do? What about that funny way that soccer players run around with their hands awkwardly reinforcing the strength of their legs? Counterweight. Balance. Power. Not sure exactly why in all cases, the palm is facing inward, but I know for a fact that I do goofy looking hand positions like that too when I throw. The opposite side of the body has to counter what the activate side of the body is doing. If I grab a disc and huck a flick in slow motion, one of the first things I do is reach forward with my left hand. As I start the throwing motion, my left hand moves back toward my body. If I pretend someone is hugging me on the mark, I throw my shoulder forward without extending my arm. Counter. Counter.
Further, if you imagine that this handballer has a disc in his hand would there be any changes to make to his form? Aside from the largely irrelevant notion of "wrong pivot foot"? I come up with exactly zero.
So, why, again, do we teach people to throw low?
To add on, if you consider the arm extended at the following degrees (60, 90, 110) from the torso, which of them provides the greatest distance from hand to body? 90, pretty obviously, for those of us who have seen triangles before.
So, why, again, do we teach people to throw low?
This is why, while I have monkey-arms, my arms seem even longer. Even when I throw from a lowered body position or a sub-90 angle, I am extending my arm as far as possible at the point of release.
So, why, again, do we teach people to throw with their elbows pinned to their sides?
If you've played against/with me and wondered "how does he throw like that?" the answer is in this handball picture. The answer is in how infielders throw in baseball. The answer is in "because my body works that way".
Next question... why do people insist on releasing low in the wind? Is there less wind there? Is it because they believe that by throwing the disc UP to their target that the disc is less likely to fly up up and away? Is it because they can't actually throw well so they need to get low and throw hard hoping that it will balance out?
Next question... why do people say "think about the wind before you throw"? Isn't that like "looking at the defender instead of the offender" when you throw? Doesn't that just mean, in a very literal sense, that the defender/wind is in your head? Isn't "so I don't have to think about the wind when I throw" why warmups that involve game-time throws in the conditions of a given game are important?
Next... Nope. I'm done. That was a lot of rambly-type typing just now.