Believe me, I've seen some really poor mechanics in my day.........everything is relative so to say that this young lady has decent mechanics (for her age) is a compliment. If you look at the most recent slow motion post you can see there are still a few things to clean up before she wins a national championship. For example if you stop the video at release point it appears that she is a slight bit over closed both in hips and shoulders.......the deceleration of rotation could be better which would probably result in a less closed position at release and an even better transfer of energy.
Now having said that.....again, this young lady is doing very well. I know for a fact that my own DD was not at this level at the same age.
Rick, This hip-being-over-closed suggestion rung a bell for me. In pitching with my DD just last night, her hip was over-closing at times resulting in her arm hitting rather than brushing her hip. I found that part of the problem was that she was not opening the hips sufficiently and she was trying to close them too much just prior to release. What helped her was telling her that while she should remain square as she initiates her leap, as soon as she actually pushes off the rubber her hips should fully open. Further, I told her not to focus much, if any, on closing the hips, as they would close somewhat (40-45 degrees) on their own from heal-plant to release. It seemed to help her. Of course, until I got advice from you and others, DD was tilting a lot toward 3B to avoid hitting or brushing her hip. It's been a tough thing to break. We're making progress, however.
As you stated, Rick, I think this young lady is doing very well. Wish my kid would have been pitching and playing at that age!