You can see a photo of an aftermarket slip clutch on the AgriSupply catalog website. They cost about $70. They are basically simple devices comprised of adapter shafts to connect between your
PTO shaft and the implement and two spring compressed clutch plates. The idea is, as the name implies, that a clutch, in this case two plates compressed by a series of springs on the casing, transmits power to the implement. When there is an obstruction, the clutch slips and prevents the shock load from being transmitted to the tractor or absorbed completely by the implement.
A key point in using a slip clutch is to make sure it is adjusted properly to begin with (ie it does in fact slip and you don't have the springs set so tightly that it won't) and also that you periodically loosen the springs to allow the clutch to slip so that the two plates don'g rust together.