Covers (Side Cover/Forearm Block)

Raise your hand so that the heel of your palm or curled knuckles is braced against the back side of your head.

-When defending against a hooking punch, your arm is maximally bent at the elbow so that the wrist of your blocking arm is close to your ear.
*technique example

-When defending against a straight punch, you can use the same technique as above adjusted by slightly turn in to protect your face. Another variation is to place the palm of your blocking hand on your forehead.

Here Andrei Arlovski covers against the jab, cross

Covers can also be used to block against hooks to the body.
Tutorial