What is it?
Striking the cue ball below centre causes it to rotate backwards as it travels forwards. Cloth friction will convert this motion into a skid and eventually a roll. A stun shot occurs when the cue ball starts off rotating backwards and is just about to begin rolling as it contacts the object ball; at this point it is skidding on the cloth. (It is reasonably easy to see the point at which the skid turns to roll as the forward motion of the cue slows before this point and speeds up after it).
The main advantage of stun is that it kills the motion of the cue ball (when the contact is full ball) so that the final position of the cue ball can be accurately predicted. Also, as the cue ball will initially travel faster than a plain ball shot, there is less chance of any roll-off. Furthermore, it is often easier to play the shot firmly than delicately.
Most club players use stun effectively. A typical shot would be a straight pot red into a pocket (to leave a cross loser) with the red near the top cushion and close to the corner pocket.