YouTube - Roger Gracie vs. Jon Olav Einemo (3 of 3)
start watching at around 3:45
the op described where the opponent ends up belly down - that doesn't happen to roger, but the concept behind the escape is there and Einemo gets out the back door.
my first instructor taught me this escape - and him having 10+ years of experience, i don't think he would have showed this escape if he thought it wasn't a legit technique.
my current instructor immediately recognized this escape when i tried to do it, and he said, "hey no leg locks today" because he thought i was setting up for a leg lock. and he's been doing bjj even longer than my 1st instructor. again, if this was some fluke move or something he felt i shouldn't be using, he would have said so.
in my eyes, the escape can definitely look sloppy in execution since a scramble is usually involved, but i wouldn't say it's a crappy technique that only works on people less experienced than you.