When practicing on a very slick surface the integrity of the stance is under attack as there is nothing on the ground to hold onto. In this situation the only thing keeping the shape of the stance and the upright position is the action of what we call “Dangjin” or arch/crotch power. “Crotch Power” is certainly an attractive term on it’s own, but in this case it is very laborious.
The legs must not only hold one upright properly, but also must hold themselves from slipping outward. In this practice with a lot of effort the stance can be maintained, until Fajin as you can see at the end, just shakes that rear leg loose. Losing three inches can lead to losing three feet very quickly and a badly pulled groin muscle.