How to "Bump-Fire":
With your rifle at your side, pointed downrange, grasp weapon firmly by the handguards with your left (weak) hand. With your right (strong) hand, stick your trigger finger into the trigger guard, but do NOT pull the trigger. Your trigger finger must be straight and stiff. Curl your other fingers and place them underneath the rear of the gun, to support the gun's weight only. Do not grip the gun at the rear.
Now, keeping your right hand held stiffly, wiht your trigger finger extended, PULL the gun forward into your trigger finger with your left hand, maintaining forward pull always. The gun will move forward until the trigger contacts your trigger finger. The gun will fire and recoil will move the gun to the rear. Because you are continuing to apply forward pressure with your left hand, the gun will move forward again, causing the trigger to hit your trigger finger and repeat the cycle.
With a little practice, you can empty the magazine in simulated full auto in one continuous string. This is what all those junky "Tac-Triggers" are trying to simulate, but the joke is that they are totally unnecessary, because you can duplicate the effect without them.
-Troy