Actually my slip clutch was built integral with the mower at the factory. It bolts onto the round keyed shaft on the gear box and as you can see, it has a "U" joint on the piece that goes between the clutch plates. That piece has a square shaft on it that the round shaft slips over.
The one that I pictured above is an aftermarket unit. You can usually buy the to fit the kind of shaft you are running on your mower.
Below is a picture of mine when I was replacing the clutch discs in it.
You can see how a slip clutch works. That middle round plate is on the end of the square drive shaft. The two big outer plates have a clutch disc that rides against either side of the plate with the shaft. The long bolts and springs clamp the two outer metal plates against the discs and the middle plate.
The springs regulate how much pressure the discs put against the middle plate that drives the mower shaft