The  above videos are small representations of one of the vital training methods of Ving Tsun Kung Fu, known as Chi Sao. On the left Sifu Moy Sum Tung rolls with his todai. As you can see he executes a couple of trapping hands methods. On the right Si Hing Konah rolls with his Si Dai(Younger Brother). They are silmutaneously exchanging energy, shifting eachothers momentum and pushing back and forth.