One of the few how-to videos by a guy who knows his stuff, this video by George Matheis from Modern Combative Systems  is a valuable aid to using the finger sap. I do feel he wears the sap to far up on his finger, I bet the strap was tight for his fingers - I advocate having it down further just like wearing a ring. Less likely to fly off and retain, and more naturally "slap" the target with a full hand and not a finger tip.