I've seen small aquarium powerheads that are free spinning on the shaft, to act as a clutch. if you pushed it inwards it meshed(the inertia or pressure of the inleted water keeps it meshed?) , but if it cant move the water it slips up'n'over the notch that meshes it...
I think the Eheim1250's do slip but only if you give them abit of gip
, or they loosen up with use, cause mine felt solid when I looked at it when new, but has started making noises now...