I've done them and it hasn't stunted my growth and i've never heard it doing it. You jump off says stairs,bleachers or wherever your at just off a platform about 12-18'' off the ground lightly jump off the platform so u land on the ground and upon contact as fast as you can explode up as high as you can and repeat for desired reps/sets. after you land and jump you walk back up to your platform and do it again. I've read some luke lowry stuff and he says you can't have a 40'' vert. if you can't land from 40'' so you want to increase the hieght ur jumping from this shocks your legs and results in an increase in vertical leap. Hope this helps