To break your line or to dislodge or disengage a fish if you have determined that the fish has been foul hooked.
To properly break off you should lower your rod tip in to the water. When you rod is now point down and straight hold your hand on the line and pull your back while maintaining the same tip down position. Breaking off in this way will guarantee that you and those around you will not get a hook or weight flung at them as the tension of the line snaps.
Trying to jerk your rod above your head or side can cause the line to snap at tension and to fling a hook, a fly or a weight at your or someone next to you.