bkdavy is correct in my opinion. A snap roll is a rudder - elevator maneuver, and shouldn't need aileron input. Some models might need a little aileron in the same direction as the rudder to induce the roll in the right direction. Due to the torque of the propeller, it's easier to snap to the left. Also, snapping too violently causes the model to loose too much forward momentum and thus when you stop the snap, the model will have to build up speed to fly again. The optimal control inputs are such that you still continue flying after stopping the snap.

-tychoc