Use for "Technic Coupling, Tight" and "Technic Coupling 3,5-6 Ncm"
This gear can only handle a certain amount of torque. Above this torque the gear starts to slip internally. This pervents damaging the motor when the some part of the construction after the gear gets blocked.

Lego uses these gears ofter in constructions where there is a end of stroke that blocked the motor from turning, for example a winch or other string based designs