What are anti-knockback springs?

Anti-knockback springs are mainly for race use. They will avoid any pad knock-back after high-speed cornering.

The knockback can be caused by the wheel bearing flexing under an extreme load during long high-speed turns. During turn, it will cause the rotor to push the pads/pistons inward resulting in a softer pedal feel until the pad touches the rotor again. 

With the springs in place, the pistons and pads will be pushed back against the rotor after the turn and avoid any slack in the pedal feel. 

For street use, anti-knock back springs will result in slightly more pad wear. If you experience pad knock back on the streets, you should probably look into doing more track-days!