Lock-out vs. snap-back
IntroIn a lock-out punch, the arm is thrust out to about 98% of the elbow's full extension, and then the arm is held in isometric contraction, motionless, for a split-second before it is retracted. In a snap-back punch, the arm is thrust out in the same manner as the lock-out punch, but it is instantly withdrawn without any apparent stoppage when it reaches its focus point. Both punches impact the target using muscle power with the mass of the body behind them, the difference comes after contact is made.
DifferencesIn the lock-out punch, the muscles that contract to thrust the fist into the target are contracted isometrically to firmly connect the arm to the torso, while protecting the elbow joint from injury due to high speed hyper-extension. Contact time is extended for a moment. In the snap-back punch, the arm tense at contact and then instantly relaxes and retracts; contact time is extremely short. Both types are appropriate for different circumstances, targets, and desired outcomes.
The lock-out punch is useful for beginners to learn punching motions. Most patterns and one-steps use the lock-out punch. For sparring, the snap-back punch is usually more effective. Georges Carpentier, in his book The Art of Boxing, says that a punch should move like a whip; it should quickly lash outward, strike your opponent, and be back to guard before the opponent knows what hit him.
If your opponent is charging forward about to run over you, then drive a lock-out punch into him or her, with a firm connection to the floor to take the impact of impaling him or her. To drive an opponent backward, bury a lock-out punch into them with deep penetration. To hit an opponent who is relatively stationary or moving backward, use a snap-back punch.
Locking-out a punch leaves you vulnerable to counterattacks for a relatively long time. If you are dealing with a person who knows joint locks or wrestling techniques, always snap back to prevent grabbing. During competition locking out a punch wastes too much time, you need to snap back and quickly attack again and again until "Break!" is called.