The last chapter showed the combination of zheng and qi. This one is about the moment that combination actually goes off. Sun Tzu draws two pictures.
The same force produces different results depending on how it is poured out. Power gathered over time and the instant that power breaks loose are separate stories.
激水之疾，至於漂石者，勢也；鷙鳥之疾，至於毀折者，節也
ji shui zhi ji, zhi yu piao shi zhe, shi ye; zhi niao zhi ji, zhi yu hui zhe zhe, jie ye
Rushing water carries boulders away because of momentum. A hawk strikes fast enough to break its prey because of timing. (The Art of War, Momentum, translated directly)