F=MA , A=acceleration

We cannot change our body mass much. But we can change the moving speed. And thus force is generated.

To relax our body is correct. Then it will function more like a cane than a stick. As the cane is soft( symbolises relax ), therefore more portion of it is moving compared to a stiffened stick, during beating. And speed builds up.

To advance from there, would be to tense up the muscle at the critical instance to create a push from behind, to build up not only velocity but acceleration( velocity per time ). The muscle strength and reflexes are playing the role here.

To undergo the training of these two elements, is called 'kung' training, from the word 'kung fu'.

Most of the time, simply physical training would not get us far. Which is why some athletics take drugs to help them in daily training.

In the old days, we use mental power to help, instead.