Now this is just my opinion, but it looks like your athlete did not "touch and go" on the box.
It looks like he had a fairly hard impact with the box (shearing force, compression of vertebrae, etc.) and then wobbily came out of the lift.
What happened to the old days of Frenn picking his feet up while on the box to "explode" off? I am not a fan of that either, but why the change?
I am a fan of straight weight. Own a certain weight by mastering the mechanics of the lift, utilizing the same form you had with the PVC, then the barbell, then 135, then whatever.
If form changes, you did something wrong. It may be boring, but guess what? You are never kidding yourself or the trainee.