Dear Kiki,
The purpose of applying such forms of ligature around the base of the penis and/or gonads, is to restrict the sanguinary flow in order that the erectile organ should be able to increase and maintain its dilation as well as promote greater rigidity and sensitivity in the glans.
I hope this clears up any confusion you might have had regarding the use of cock rings.
Tao
PS : Sorry but I'm really fey tonight. Lucky Carnavon got in first.