Must Be Loose
Looseness is central. Without looseness there is no fast and no slow. Without looseness there is no hard and no soft. Looseness is the key to flexible hard-soft-fast-slow movements. The looseness comes from the flexibility, openness, softness of the hips, of the kwa.
The link for slow, fast, heavy and light movements is looseness. Power comes from looseness. Throw the hard strength away.