When riding 4 stroke bike and u down shift really quickly,ever notice how the rear wheel tends to lock up a bit this problem is more apparent in the bigger 4 sroke bikes coz of the massive amount of engine braking the motor develops and the same problem is almost non existant in two strokes coz their motors hardly have any engine braking.Which is why there has been such a big fuss over the switch from two to four strokes in motogp,coz riders have had to change their riding style.

What the slipper clutch(also called back torque limiter) does is as soon as it senses the rear wheel locking up or in more basic setup's as soon as a preset amount of back torque is reached,it disengages the clutch from the engine ,though not fully, so that all the engine braking generated by the motor is not transferred to the rear wheel which in turn prevents the rear wheel from locking up.

until recently these type of clutches were only available on higher end italian bikes like the Aprilia's and the Ducati's but starting from '04 the new ZX10R's have got the slipper clutche's as well heaven knows,, they need it. dunno bout the other Japanese bikes.