Early tractor PTOs where especially, um, fun. No built-in overrunning clutch and no "live" PTO means the inertia of spinning implements could turn the PTO, and

Early tractor PTOs where especially, um, fun. No built-in overrunning clutch and no "live" PTO means the inertia of spinning implements could turn the PTO, and thus the drive wheels. Nothing like standing on the brakes while the tractor just...keeps going for a few feet. Fortunately overrunning clutch attachments are cheap to add.