The toe hold. An underutilized submission in MMA if you ask me.
The general idea behind the toe hold is that if one hand grips the outside of an opponent's foot such that the bottom of the fingers touch the bottom of the opponent's foot, then by using the other free hand to snake in front of the ankle to grab the wrist of the hand grabbing the foot you can exert severe pressure.
It sounds a little more complicated than it is, so think of the grip people use for kimuras. It's essentially the same. The difference with the kimura (besides the obvious) is that the wrist-on-wrist grab is used to lock the arm in to exert pressure on the shoulder. With the toe hold, the wrist-on-wrist action exerts pressure on what the actual hand is grabbing rather than a knee or something else. In either case, the grip locks the opponent in. With the toe hold, it allows the tori (a judo term meaning the person applying a submission) to use the angle of the grip to force the uke (the person receiving a submission) to tap or have their foot turned too far inside.
This also should be a lesson to everyone in why gripping is so important in martial arts. Without the right grips, your game will always be full of holes.
In this video, Leben shows the toe hold from full guard. Don't bother with the how he opens the guard or fakes the pass. Just pay attention to his grip and how he exerts force. You don't even need to twist the uke's leg like he does, but obviously it helps. For a more classic example of how a toe hold is applied, check out Frank Mir's destruction of Tank Abbott. He uses the oma plata to set it up, a traditional but clever fake out. The submission is about half way in.