Triangle Choke

Here is a good video and a good description of the triangle choke too.

1. Get wrist control on both hands.
2. Push right hand across his body to his belly button. This makes room to start the triangle choke.
3. Open guard. Raise your hips up and throw your right leg over his arm and shoulder. Lifting your hips to his head is extremely important. Lock with that leg on bottom (doesn't have to be great lock yet).
4. Get head control with your right hand so he can't posture up easily.
5. Open you lock. Your left foot goes on his hip with the knee pressuring close to his body just like in an armbar. Your right leg pressures downward; grip your right shin with your left hand. Try not to grip your foot, since you could hurt yourself and it is more loose. This helps keep his posture broken still.
6. Create an angle to the right so you are perpendicular to him. Do this by pushing with your left leg and pivoting on your hip instead of sliding.
7. Lock the triangle with your left knee over your right ankle. This is like a figure-4 or a triangle.
8. If his trapped arm is in the way, raise your hips and drag it across your body.
9. To finish: Bring your knees together and your left foot to the ground. Pull down on his head too and remain perpendicular.
10. Unconsciousness...

You can set up triangle chokes in many ways besides this basic method. One way is to overhook and shrimp out a little bit, push his free arm out and throw your leg over, and finish with the triangle choke. The shin-guard and rubber guards can be used to set up triangles too. Triangle chokes can be used in combinations with armbars and omoplatas as well.