A very painful submission to the ankle and knee! I have used it and had it applied to me many times. I will try to describe as best I can.
The applier(sp?) of such a hold takes the foot of his opponent and points the toes inward braced against usually his ribs . He then underhooks the foot by the rear (heel)and forcesit outward against the grain while usually baring the knee of the same leg the hold is being applied with his own legs wrapped around the victims to ward off rolling out.
Consider it massive forced pigeon toeing!
No that is a legbar.That manuever forces the knee against the grain. Using the same picture, if Ken were to cross his legs over Bas' knee and underhooked the foot with his forearm and lower bicep (and applied the previously stated pigon toe) it would be a heel hook.