A slip clutch will slip when the torque on the pto shaft exceeds a given thresh hold. This is done to prevent damage to the tractor or implement.
If a slip clutch is not used then a shear pin is often used. The pto shaft is held onto the implement by a single bolt of a given hardness. When the torque on the shaft is exceeded then the bolt snaps protecting the tractor and implement.
It is not normal for the torque of an implement to exceed its rating except if it has encountered a problem. Some examples would be: a Bush hog hitting a stump, a rototiller getting a rock stuck in it, a baler trying to bale a large branch.