Technically in full scale aerobatics the snap roll is defined as an auto rotation around the yaw axis with the wing in a full stalled condition. It's a horizontal spin. The technique is to raise the angle of attack abruptly to cause a stall and at the point of the stall apply full rudder in the direction you want to go. Ailerons are cheating. You can get a high speed roll that degenerates into a snap, but ideally the maneuver is done stalled with rudder only. The hard part is getting the AOA to go high enough at high speed to get the stall.
HTH
Tom