"When people are earning a new physical skill they often apply way too much muscular tension, which will naturally reduce as they become more proficient. Let's take a left jab as an example; a raw beginner may tense their bicep at the beginning on the punch and keep it tense all the way through, which is actually counter-productive because it means the tricep has to work harder than it needs to in order to extend the punch. Same applies to other muscle groups. With more training, the boxer learns to relax certain groups and their punch speeds up and becomes more efficient.
However, the mechanics of a jab or cross are distinct from those of the so-called "ballistic strike", which is also referred to as a wave strike, a whip strike, etc. Linear punches work by aligning the skeleton behind the strike, which allows an efficient transfer of body weight into momentum (supported and directed by muscular tension and extension). Ballistic strikes operate by allowing both flexors and extensors to remain relaxed, as Vieux Normand explained; the arm (assuming we're still talking about punching) is effectively a dead weight, unsupported by the muscular tension required to maintain linear skeletal alignment. It's like a medieval ball-and-chain, as opposed to a battering ram.
The intermediary step would be something like a shovel hook, the main difference being that a hook is typically thrown with enough tension to retain skeletal alignment (i.e., to get the weight of the body more-or-less directly behind the punch) whereas the ballistic version is thrown with very little tension, the power being generated by a weight shift from foot to foot and delivered by a "dead arm" swing.
Frankly, both methods - the more orthodox, tension and alignment-based punch,and the ballistic punch - generate serious power. The ballistic punch doesn't require as much of a chamber and is optimized for sucker punch scenarios, which is (IMO) probably why it was originally favored in Systema (re. the historical connection between Systema and executive protection work)."
-dLlR
