IMO It's not. Lots of minutia being discussed here for something when at 100% efficiency accounts for a maximum of 10% of her total velo. So if she is already at 70% efficiency on her drive, how long and often would she have to work to get it to 90%? And for what? An additional 1-2 mph tops. And have I mentioned that there is no pot of gold at the end of this rainbow called girls softball?but I'm not sure if the "juice is worth the squeeze" when it comes to working on this.
if your DD is throwing 58 at 12 yo she's already doing a lot of things right. I wouldn't sweat the minutia of "fully engaged hamstrings......... quad dominant vs glutes............pelvic thrust...yada yada yada". Use that time to instead work on spinning the ball, hitting corners, and an awesome change up. Those will benefit her 1000 times more than an extra couple mph. That's going to come naturally as she gets stronger anyway. That and then some.
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