Snap rolls are a roll that is off axail (ie they kinda corkscrew) and are faster than ailerons alone. First, before you attempt a snap roll be sure you have the right tool for the job, that being your plane. Your plane should be an aerobatic plane with a strong wing, because a snap roll puts lots of stress on the center joint of the wing, can be up to +/- 10Gs! Also you need lots of elevator and rudder throw. There is 2 catagories of snap rolls (Positive and Negative), also called inside and outside. The positive or inside (can be left or right) snap is when there is up elevator and the snap corkscrews up then back to level, negatives or outside snap is with down elevator and the plane corkscrews below the flight path and back to level. During the postive if you were in the plane you would feel postive G's and negitive during a negitive snap. Now that you know what they are heres how to do them. First off you don't want to be traveling very fast because the faster you are going the more load that is put on the wing. So start with flying level at about half or just below throttle. Now pull the nose up about 10 degrees and do one of the following combinations.

The plane will perform a snap roll, multiple ones if you hold the controls. To stop rotaion release the controls. Some planes will rotate a bit more then stop. Strong aerobatics designs will stop right away.