I agree, it is not a friction slip clutch like is found on bush hogs and tillers. I think it only slips in one direction and it will make a popping noise as the chipper winds down. I think if you take it apart you will find a spring loaded sloping ramp down to a 90 degree cog that engages positively in the power on position but allows the chipper to wind down while the tractor PTO is stopped turning. These things are common on the old square balers that have a big inertia flywheel.I don't think it sips in the sense that you're thinking. I think that when the flywheel --let's say a baler--begins to "drive" the PTO on the tractor, this thing slips. In addition, when you turn the tractor PTO off, the things slips so the baler flywheel doesn't drive the tractor. I think that's what made the clicking sound when I shut off my baler and the flywheel kept turning but the tractor PTO did not. It just free-wheeled. I'm not positive but pretty sure I have this right