Snap rolls are initiated by first feeding in full up or down elevator, followed almost instantaneously with full left or right rudder.

The idea is to stall the wing with elevator and as soon as it's stalled, initate the snap with rudder. Some aircraft also want some aileron added, but usually this slops the snap up a bit.

Some aircraft will do an ugly snap roll with full aileron and elevator simultaneously applied.

Hold the settings for the snap (at altitude!) (full rudder and elevator) and most planes will then enter a spin. Neutralize controls, apply opposite rudder to stop rotation if required, elevator out of the resulting dive when sufficient airspeed is attained. What fun!

J