Dan-Gun: Knife Hand on Premium Unleaded even in Back Balance
The Soodo-marki or shuto in Dan-gun is a rather flimsy move when you try to pit it against a lunge punch or a leg. Yet the knife hand is a versatile striking weapon, with greater angles of entry and speed than the basic punches learned in Chon-ji or Dan-gun.
Creating the power with the blade of the hand is tricky. For our beginners, I teach folding for the knife hand (done korean-style with two hands held parallel away from the opponent) with a supple waist, hip turn and lead foot with heel held slightly off the ground. Then a step into a back balance with the lead foot lightly touching the ground. The swing around move is preempted by using the waist and hip to rotate sharply towards the lead foot, sitting the backside down in mid-air, and pulling the lats and arms downward - sinking them alongwith the center of gravity. This means that the striking weapons are pulled down into gravity using your bodyweight, and that their velocity is then accelerated through lats and triceps contraction. This is one of the reasons why this particular shuto is held fairly close to the body - extend it and it would be all triceps - no lats to connect the strike to the body core.
An analogy is that of the ice skater - spinning with arms out is slower. Pull the arms in and the spinning gets much faster. The knife hand pulled backwards into the body gets increased speed. Drawbacks are that you need to be closer - but I reckon this is where you'd be anyway.
Much of the force comes from the contraction of the body, rather than a swinging out of the arms. If it were only the arms, the strike would feel flimsy. Start doing the hip turn and sitting down correctly, and you start feeling a lot of boney impact.
PMA: Knife Hand Practical Applications