Imagine a woodcutter with an axe...

If he hacks at the wood using great force and muscle strength, he will rapidly become tired.
The axe head has weight and so does the shaft.
When chopping, the woodcutter must propel the axe using his body.
If he softens the tension in his own frame, he can use momentum to swing the axe.
Tension in his muscles and joints will block movement.
It is the movement that produces momentum and the greater the momentum, the more powerful the stroke.

Think of an athletics event where a hammer, discus or shot putt is thrown...

The person moves their body in an arc, allowing the hammer to pull out from the body.
Centrifugal force draws the hammer around the arc, increasing its weight and power.

They move their body around the hammer, increasing momentum but sustaining the gravity.
Then they let go.
Striking in kung fu uses centrifugal power to gain momentum, then centripetal power to come back upon impact.