Power Functions set 8293 contains these pieces http://brickset.com/sets/containing-part-6025005 http://brickset.com/sets/containing-part-6036892 and they make the set much more expensive than just buying the other parts in the set individually. By that reasoning these parts must be very special, but I can't tell what is different about them from the images compared to a regular gear wheel.
1 Answer
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
