A device used typically in radio controlled cars. The steering servo is the little device that makes the wheels turn left and right when you move the stick on the transmitter. Now, if the steering servo is connected directly to the wheels, it has a nasty side effect: it means that when you bump the wheels against something really hard, it will force the servo to turn in a way it's not supposed to. This will cause the gears in the servo to strip, which will ruin it.

So, rather than connecting the servo directly to the rods that drive the wheels, you have a lever that it attaches to that is spring-loaded that is attached to the wheels. Normally, it does nothing. But, in the unfortunate case of hitting your wheels, the spring takes the brunt and not the servo, and hence it saves your servo from damage.