A safety device used in nitro rc cars. The carburetor in the engines of such such cars has a mechanical throttle on it that controls how much fuel is being burnt and hence how fast the care is going. The throtte is connected to a servo, which pushes the throttle to make it go faster or slower, under the driver's control from his radio control transmitter. But for various reasons (like batteries dying, or a really bad collision which takes out the receiver or something), servos sometimes fail. If it so happens that your servo fails when it is at full throttle (especially if the steering servo also fails), then you have a pretty serious problem on your hands, commonly called a "runaway". A $300+ 2kg piece of equipment travelling at upwards of of 40km/h jammed at full throttle is quite amusing sometimes, but may be (a) dangerous to "civilians" or whatever else it will eventually hit (b) expensive to repair.

Hence you have a throttle return spring. The springs are set up in such a way that if the servo loses power or fails, the throttle will be pulled back by the springs into the idle position and damage will be averted. This means also that the servo has to work against the throttle return spring whenever it wants to push the throttle forward, and will brake faster when it's installed.