Rebound is the amount the shock shaft moves out naturally after you compress it (with no shock spring installed) due to internal pressure of the oil in the shock. On the track, this will present as additional resistance to shock compression and faster snap-back of the suspension after being compressed.
Here are the effects of changing the rebound:
- Car feels more responsive
- Bumps will unsettle the car more easily
- Less responsive
- Smoother over bumps
Some manuals and setup sheets recommend a certain rebound level, so if you are trying to emulate someone else's setup, be sure to also use their level of rebound, or your car will not handle as intended.
Bladder style shocks provide more rebound as they compress the air within the rubber bladder as they compress. With emulsion-style shocks, the air is blended with the shock oil, so it does not contribute to the rebound.