Bobbing is one of the basic strategies of defensive boxing, executed by slightly moving the head to either side so that the opponent's punches slip by the boxer's head. You use the slip to evade swings

Swing (boxing)

The swing is a type of hook, with the main difference being that in the swing the arm is usually more extended....

, jabs, and straight punches. It can not be used with hooks

Hook (boxing)

A hook is a punch in boxing. It is performed by turning the core muscles and back, thereby swinging the arm, which is bent at an angle near or at 90 degrees, in a horizontal arc into the opponent...

as they move on the side level. Using slips is valid but risky with uppercuts since the punch is usually too close when the defender can determine the exact line of the punch. To overcome the hooks problem, the defender usually incorporate slipping (also called weaving) with ducking (also called bobbing)