These are also known as micro-rings because they are indeed micro. They’re called tippet rings because they are attached to the end of the leader body (butt section plus tapered section) as an attachment point for the tippet. OK, how micro is micro? Well, they come in 2 mm and 3 mm sizes (that’s about 5/64 and 1/8 inch, respectively. Very small. The 3 mm size works great for heavier leaders; for example in Czech Nymphing, there’s a short heavy, red or bright green butt section and then a very long tippet section (up to 10 feet, or even more in some circumstances). The 3mm ring goes at the end of the butt, and the tippet can be changed as needed. The 2 mm ring can go on the tippet to act as dropper connection points. In addition, micro-rings are the best choice of connection between a furled leader body and the tippet. And don’t worry, they’re plenty strong–30 to 35 pounds. They come in black nickel, silver nickel, and titanium. They work with dries, too (a touch of fly floatant will keep the ring on top).
These are an European idea and they’re great. I’ve found one shop that handles them–I’m sure there are others, or soon will be. Click her to get yours: Micro-rings.