CAUTION : This technique is extremely dangerous. Some people will not feel any pain until their ankles actually break. You have been warned!
In this technique, your opponent has the back mount but has crossed his feet (something you should NEVER do from the back mount).
If your opponent has his right foot on top of his left foot, put your right foot on top of his (as shown). If it's the other way round, use your left foot.
In this demonstration, put your left leg over your right foot.
Now just lean back, raise your hips and push your left knee down. Even if your opponent has a perfect choke sunk-in, he will still tap before he chokes you out.