I have not used that one but similar rings like it. The idea is that one ring goes around the shaft and the other around the testicles. Another way would be to use the bottom ring around the whole package (Shaft and testicles) and the top ring around the shaft.
Either way, they provide a stronger erection and push the testicles forward keeping them there. I enjoy the feeling of my testicles hitting the wife during sex and it adds to her pleasure as well.