Rings are used to concentrate the blood flow on the penis, prolong erection and improve sensitivity. Many men find it very pleasant, although each physiology is different and in the end it depends on your characteristics.
They are usually placed behind the scrotum, but some men prefer around the penis. It's easier to put it on when erection is not yet complete, and sometimes it's easier to shave the area.
Precautions: Do not use longer than it feels comfortable and never more than 20 min. Do not use to prevent erectile dysfunction without visiting a doctor before. Do not use if you have blood clotting problems, or if you are diabetic.