We used to stick our inside leg out and off on really fast turns when we were downhill mountain biking. Not sure why it worked. But it did something. I think it isolated our body from feeling bumps, and it got our feet out under us, so we felt more normal as well.
With our inside foot clipped in, it was up high and leaned oddly. Being that moto GP bikes have very high pegs, I can kind of see a parallel. It had to work, because clipping in and out on a mountain bike while gravity trys to kill you, is not the easiest thing to do. So why do it?
I do remember, squeezing the seat with your legs gave more control of slides, but the bike would fishtail and slide more. Leaving your legs loose, not touching the seat, gave less feel, but more traction. Unclipping and hanging a leg out was like taking the latter loose leg idea, and going further with it.
I've noticed that gripping the seat, and floating loose over the bike, have similar effects with motorcross bikes. But I think they sitck thier leg forward and off for different reasons. Just to get weight forward for steering a bike with slack head angles.