Servo fail safe. If the radio receiver stops outputting (e.g. if the model has lost transmitter signal) the fail safe will kick in and output a known signal and put the servo or speed controller into a safe position. I've got 5 available that were last used in a robot wars robot in the 2000's! (Infernal Contraption from series 6 and 7) This listing is for 1 failsafe. Not necessarily the one photographed, but they are all working with varying amounts of stick pad residue.
eBay