A snap roll is really a spin when the aircraft is moving across the horizon. If your aircraft is set up for acro type flying, you should be able to snap roll it by adding full elevator (stick back) and full rudder at the same time. I think what you are referring to is when someone says the plane is snappy, they are talking about the fact that some airplanes will enter a stalled/yaw/rolling situation when a little too much elevator is applied..