Shown to me a couple years ago by a guy who teaches the Hong Kong police. We've been having a lot of fun with it, creates a lot of power, knocks down or back a lot of people. And obviously puts your hands right there on him. But I forget all the nuances.
Say you're throwing a straight vertical punch to the body/sternum....with your off hand you're grasping the wrist/hand of the punch to make it stronger. Sort of like doing a two hand "get away from me" shove to the chest. But you're not pushing, you're slamming it into him.
What is this technique called? What art(s) is it from? What is the proper position of the feet? Should you be sitting into your stance or leaning forward?
Any insight would be much appreciated!
At 47 seconds, you see the reinforced punch. Please note this is not my video, and I will say only that I would perform the kata this way. However, it does illustrate the reinforced punch you noted.
Please note the 'obvious' bunkai for this punch is simple - a reinforced punch. The stance is zenkutsu-dachi, feet parallel and facing forwards, about shoulder-width apart, and in the Isshinryu tradition, heel-toe alignment, rather than the deeper stances of other styles of karate.
Note also that the punch does not generate tremendous force from the hips; it is a short-stroke punch, designed for close quarters. The left hand (in the kata) grasps the right hand just behind the thumb of the right hand, covering the right wrist in a palm-down position. The right elbow is held close to the body and does not extend as a 'normal' punch would; it moves a fairly short distance.
Note also that in the kata, it moves immediately into an elbow strike in an upwards direction with the same right arm that just did the punch. These two are linked for a reason; the elbow strike develops from the reaction to the reinforced punch. One might infer that the reinforced punch is not a 'put away' punch.
Advanced bunkai would be difficult to explain in writing; but suffice to say that it exists. One example would be using the reinforced punch in a very small circular motion (nearly too small to see clearly) as a clearing movement to brush away an incoming strike and then immediately strike behind the attacker's punch, devastating when they have extended their arm and opened their ribs. Think coffee grinder movement.
Is this the movement you are asking about? If so, the best way to understand it is to experience using it and getting hit by it in training.