Question

A karate player suddenly reduces the speed of his hand while hitting an ice slab.

Answer 1

Though the speed is reduced, the contact area to the ice slab is reducing too.

But then again, the force or the energy generated by the entire body of the karate player is striking the ice slab at a slow speed, making it much more intense.

This breaks the slab.

Answer 2

To hit and break an ice slab a karate player suddenly reduces the speed of his hand while hitting an ice slab.

This reduces his or her hand's speed and the area of the contact, which slows down the speed but increases the force intensity with the previously gained momentum.

This helps to easily break the ice slab with just one hit.