Face down/belly down armbar
I don't know the "proper" name of the technique, but I have heard it called both a face down and belly down armbar. This is when you get stacked while you have already secured an armbar from the guard and you roll with the pressure, putting you and your opponent face down with their arm still trapped.
A week ago we were drilling armbars from the guard and what to do when stacked or if they try to pull away. While being stacked and rolling them, we were taught to remove the leg hooking the head, then push down on the side of the head with your shin while rolling, and finish it on the ground with your shin pinning their head to the ground.
I can't remember where, but recently I saw a MMA match (possibly pride) where someone locked on the armbar from the guard and got stacked, rolled with it and finished face down. They kept their leg hooked around their opponents head the whole time though.
I know there are lots of different variations on just about every move there is. Are there any advantages to keeping their head hooked with your leg opposed to driving it into their head? In terms of disadvantages, I hurt my toe pretty badly when I forgot remove the leg hooking the head, although if more careful I'm sure that can be easily avoided.