Properly positioned guards and shields that undergo regular maintenance are important safety features that help prevent entanglement with PTO drivelines. However, guards and shields alone are not enough to prevent an entanglement incident. Operator awareness and constant caution are critical to avoiding power take-off (PTO) entanglement injuries.
The major components of PTO systems. Reproduced from Grisso, B. (2009, Machinery Safety on the Farm, Virginia Cooperative Extension http://pubs.ext.vt.edu/442/442-092/442-092.html)
The master shield of a tractor, located at the rear of the tractor over the PTO stub shaft, is the first shield along the PTO driveline. When installed and used properly, the master shield prevents the operator from coming into contact with the universal joint of an implement driveline, as well as the stub shaft. When operating a PTO-driven implement with your tractor, you should encase the shaft in a driveline shield: a plastic or metal casing supported by bearings at each end of the shaft. The bearings allow the shield to stop spinning if someone or something comes into contact with the driveline while the shaft inside continues to spin. The ends of the driveline shield are bell-shaped to cover the universal joints of the shaft. Because universal joints are irregularly shaped and prone to snag objects, operators should never modify the bell-shaped shield in an effort to make maintenance, greasing parts, or connecting the shaft easier.
It is very important to perform a walk-around inspection of your tractor before and after your work day to make sure that all necessary guards and shields are securely in place.
Resources
Howard J. Doss, with the Michigan State University Extension, provides some keys to PTO safety. You can access this information at http://nasdonline.org/document/1295/d001094/shield-yourself-from-pto-dangers.html.
Other documents on PTO safety can be found at http://nasdonline.org.