ummmmmm a cockring goes around yer nuts too- they dont really work unless you constrict the circulation from back there- you want blood to be able to get in easy but have a harder No pun inteneded) time getting out- I think you will have a ahrd time finding a shaft only cock ring.....
Yeah it definately goes around the shaft and the nuts.. Its got to be tight so that it constricts .. keeps it much harder and harder to nut. You should be able to put your finger and everything else in while soft but not so loose to fall off. Try the rubber or leather ones first.. Easier to use
I havent really seen anything on just the shaft. Dont know if that would really be comfortable or easy...