A slipper clutch is a mechanical component that is capable of transferring excess energy in to heat via friction between the clutch pads and the clutch hub. In simple terms, this clutch is supposed to partially disengage (that is, to ‘slip’) until the motor’s speed matches with the car’s speed upon sudden braking or jumping.
Slipper clutches are often found on higher end offroad RC cars. Note that too much slip and you can overheat the slipper. On the other hand, when getting too tight it can do damage to the spur gear!
In the case of extensive slipping, note that if the adjusting nut spins while the gear turns and slipping occurs, it is the differential that is slipping. If the nut does not spin, it is the clutch that slips.
Ball diff adjustment can always be done without the need to open up the gearbox. You can tighten or loosen its hex screw after removing the dog bone from one end. Be careful though – over tightening can break the screw and render the diff useless.
