We covered the D'arce choke plus several defenses in class on Saturday, and we wanted to make sure you guys see how it's applied in a real-time tournament situation. This is a very common maneuver, particularly in no-gi, so understanding how to prevent from being caught in it is critical.
Here's a sample video of it being applied in action. Notice that the set-up occurred after opponent X passed opponent Y's guard and established cross-side. Remember your hand positioning on the bottom, and how to trap your opponent's arm should he decide to attempt the D'arce by weaving inside your top arm.
As with everything in jiu-jitsu, timing is a key element to both your offense and defense.