Today, I went to the beach. I had two big water guns. While I was filling up one with water, I left the other floating in the water. The waves were pretty strong. When the water hit my leg, of course, it wouldn't hurt me. However, when the wave pushed the water gun, and the water gun hit my leg, it hurt very much. There are two aspects to consider:
1) If my leg in the water is like an opponent and I am like the water, I will just glide through, and the opponent won't be able to stop it as there is nothing for him to hold on to, and there is practically no resistance.
2) The water gun didn't move itself, it was moved by the water. In order to make the energy in the wave to do some damage (to hurt my leg), that energy cannot become dispersed. Through the water gun structure, the energy stayed focused. On the other hand, my leg stopped the water from moving forward as there was something I was able to hold on to.
In life, there are so many things related to taiji (yin and yang).