The chamber-extend-retract action is more commonly known as "snap." For maximum power, before executing a punch you must chamber (coil, cock, pull back) the punching arm so it has more distance over which to apply power to the punch. Incorporate the chamber into a feint or block so it does not telegraph your intentions. Execute the punch and allow the arm to reach its full extension. Even while free-sparring, do not stop the arm short of full extension but adjust the position of the body so the punch terminates just short of its target. The only difference between a punch that stops short, just touches, or destroys its target, is range. Range is discussed in more detail in the free-sparring topic. Immediately after reaching full extension, quickly retract the arm to the on-guard position to prepare for the next technique. The final velocity and a large portion of the power of a technique primarily come from the snap of the elbow or knee.
An example of this snapping action is the grabbing action a snake handler who milks venomous snakes uses to grab the head of a snake. To keep from getting bitten, the handler must move very quickly and be prepared to grab the snake when contact is made. However, the handler must also be prepared to retract instantly the hand if the grab cannot be competed. To have any chance of grabbing the snake and to prevent receiving a bite, the handler must commit to the grab while simultaneously snap the hand back if unsuccessful. To executing a punch, you use a similar action. You commit to the punch, plan on it hitting the target with full power, but also plan on quickly retracting the hand if it misses the target or hits the target.
Snapping a punch seems contradictory. You are thrusting the fist forward as quickly and powerfully as possible while simultaneously planning on retracting it was quickly as possible. Adding snap to a technique is difficult and requires much mental and physical training to achieve.