A slip gun is a revolver which has been modified to disconnect the trigger from the hammer, so as to cause it to fire by pulling back and releasing the hammer. Often the hammer spur is lowered so the gun may be fired by wiping one's finger across the hammer. It is different from fanning because in fanning, one hand holds the gun and pulls back the trigger while the other hand knocks back the hammer repeatedly. Slip shooting is a little slower than fanning, but more accurate and practical since only one hand is needed.
Slip guns were used for various types of rapid trick shooting in which the ability to instinctively rapid fire was crucial.