When you punch with vertical hook punch, if your opponent give you a hard block on your forearm, his arm will hit on your "inside" blood vessel. You don't want that to happen to you.Do you hook punch with a horizontal or vertical fist?
This is why when you tough your arm with "3 stars", you will hit the
- outside surface,
- outside edge, and
- inside edge.
You will not tough your arm by hitting the "inside surface".