The Swivel Punch
This is a punch delivered with a vertical fist accompanied by momentum and full body weight. This punch was developed by U.S. Marine Corp Close Combat instructor Charles Nelson. Charlie who weighed no more than 130lbs was known for his powerful swivel punch, one time he dealt with a 220lb plus karate man who came to Charlie's school in New York and challenged him to a match fight. Charlie gave him a NY telephone directory and asked the man if he could first demonstrate his swivel punch, the man agreed and held the phone book against his chest as Charlie proceeded to punch him clear across the room. The man picked himself up, bowed graciously and left.
Here is the legendary Charlie Nelson following up with his swivel punch after preceding the punch with a chop up and under the nose.
Method of practice:
This punch can be used as a pre-emptive strike by itself or in combination proceeded by a chop to the bridge of the nose, swivel punch to the heart/solar plexus it's primary target, followed by a sharp knee to the groin. This could be used to a frontal assailant or to someone slightly to your flank as if you were approached from the side.

In this example I am punching with my left hand and my right lead foot is moved in a circle to the out side of your opponent's right foot as you swivel your hips around in the direction of your punch transferring your entire body weight onto your lead leg. Punch with a vertical fist as if punching through the centre of your opponent's chest. The sharp swivel of the body and the snap of the hips a split second before the punch is delivered off a recoil from the shoulder allows the full transference of body weight through the target. The mechanics of the punch are quite similar to Peter Consterdine's double hip vertical punch.
Here are a couple of examples of the swivel punch in application the first as a pre-emptive strike and the second as a reaction to a double grab by seizing one hand and punching the solar plexus with the other.