"Snap" is generally a shortened form of "Snap Roll". A snap roll occurs when one wing tip stalls before the other, causing the plane to roll over. Depending on the characteristics of the plane, it can occur very quickly, hence the "snap" term. Its generally executed by slowing the plane down and then applying full elevator (up for a positive snap, down for a negative snap). Adding a touch of rudder in the direction of the roll really makes it happen quickly.

Its a stall maneuver, as one wing tip must stall to start the rotation. If it occurs on take off because of over application of elevator without sufficient airspeed (beginner mistake with a highly aerobatic plane) or on approach while trying to slow the plane down to much for landing, it can be fatal.

If you're planning on it, it can be a very dramatic maneuver. If you're not planning on it, it can be easily mistaken for a radio hit.

Some planes are notorious for snap rolling every easily (eg the CAP 232). Others are very tame and controlled (eg. the EXTRA 300 series), and others have to be forced into it (eg Edge 540).

Very few trainers will do a snap roll without really forcing it.

Brad