Sometimes when you pass the guard of your opponent, they get to their knees in the turtle position. Walter Cascao Vital of Cascao Jiu-Jitsu has a solution for this situation.
Often times when in the turtle position, since your leg is close to their shoulder, they will grab for a single leg. Take advantage of this to complete an armbar.
Cascao tells us just how to do this:
http://www.youtube.com/watch?v=nbLICLBOROo