I'm not trying to troll you or give you a hard time, I'm honestly having a very hard time identifying what it is you've isolated as the key factor (caused by the hyperarch theory).
It is difficult to see initially. Took me years to isolate it. Once you see it, it is quite clear. Again focus on what is generating the forward movement.
- Girl on left is moving due to mid section thrust, girl on right is moving forward using her rear leg.
- Girl on left's upper torso aligns with her rear leg quickly (thrust), girl on right's upper torso doesn't align with rear leg until much later (late push pattern).
- Girl on left passes through the sprinters "h" position, girl on right does not.
There is enough visibility of the girl on the right to see that her rear hip remains bent while she is moved forward with her rear leg, where as the girl on the left is moving forward because of her straightening rear hip.
This doesn't make it easier to see, but it explains the effects of lacking rear hip thrust.
We've been conditioned to "teach" that good "h" position when it is lacking. I am making the argument that the "h" position should be a no-teach as it is a naturally occurring characteristic of someone with a good functional posterior drive chain. If you teach it, you are faking it, and if you fake it, it will be even harder to find the underlying issue that was preventing it to begin with. I wager the elite pitchers (just like elite HS sprinters) never needed "h" position training. It just worked for them from the get go and got better with time (strength).