Wasn't taught formerly in a class as in "This is how you do it" Was something I picked up diving with Fred and Danny. Saw them do it and tried it out, and it made a bit of a difference. With a single stage that is mostly empty it rides just a bit better when doing a kick dive.
In large passage, there is no real possibility of damaging anything. One wouldnt be doing it in smaller more restrictive passage, just as one wouldn't be using a full frog kick. Lets be thinking divers eh?
I have never seen either of those guys ever do that and I would guess I have around 100 dives or so with Danny alone. Are you sure you didn't seem them clip to their crotch ring which pulls the stage in?