i guess at the end of all of this, my take home point would be: don't look first to technique for anything running, jumping, or sprinting related. First look towards having fun and getting the reps/work in.
by reaching for technique first, what we think may be magical, could actually be poison. So just avoid it for a while and just get natural improvements via fun, hard work.
if one is going to try and change some technique, just try and change one thing at a time... also if possible, try to avoid getting someone to focus on cues while they are actually performing the main movement! instead, use drills that would subtly change/improve these things, perhaps without the subject/athlete even realizing it could be helping their target movement.
when coaching technique etc, i think it's very important that the one coaching, knows that the athlete will try and correct as much as possible, so getting them to fix things WHILE THEY ARE PERFORMING their main movement, could have them overthinking at literally every stride, runup, jump, etc. This can become toxic .. and IMHO, it's best to try and avoid getting someone thinking about this stuff at all. The best way to do it would be to utilize drills outside of the main movement that aim to correct, and then allow the athlete to perform with a clear, focused mind.
safe, progressive, fun, free/clear/foused mind, & somewhat frequent (not every day, rest/relaxed/light days are important) challenging training for children and barely teens, (and most adults) is the way to go.
quick 2 cents/recap from my perspective.
peace!!
