STM are an Italian manufacturer of slipper clutches. What is a slipper clutch? As the name implies it is a device that enables the clutch to slip - but only on down shifts, where excessive engine braking can cause the rear wheel to lock. This device will let the clutch release when there is too much engine braking - i.e. when heavy braking and down shifting rapidly. This means that better control over the rear wheel is maintained,and greater stability of the bike. Fitted as standard to high spec racing bikes, these are now available for a select range of sports bikes.