Generally that goes into the line of defence known as arm movement.
You can basically block with any part of the arm:
- inside and outside of glove
- inside and outside of forearm
- inside and outside of elbow
- outer arm (inside protects your body and head automatically)
- inside and outside of shoulder
It depends on your proportioning as to how you like to hold your hands.
Text book guard your hands aren't actually pinned to your chin. When you want to block numerous shots you can raise both hands in a peek a boo style. If you want to block one shot at a time then you only move one arm.
Sometimes you'll feel the punch go through you. You'll have to gauge what your opponent has. You need to be able to move with his shots and at the right time so as to take the sting off them.
Some guys can block everything, others feel the punches go through them. If a guy is doing that to you, you'll have to block and soak up the shot but make sure to make the follow up shots miss either with foot or head movement.