In reply to davidbeynon:
The centre is a triangle made of tightly coiled and sewn tape, to which the cord is attached. The outer segment is another layer of tape covered with a rubber sheet more akin to a climbing shoe sole than a bicycle inner tube. This outer layer generates excellent friction on sandstone or gritstone.
You place these things like a large chock, and set them with a brief pull to engage friction between the outer layer and the sides of the crack. If the cord becomes loaded in case of a fall, the inner wedge is forced down, actively pushing the friction generating layer outwards against the rock.
Traditionally, a similar effect was achieved with a flattened figure of eight knot on a bight, which you can place so that the lowest loop catches against a constriction. Load the bight, and it will partially roll the knot, causing it to widen and jam.
The Ufos are much more conveniently placed, can expand by a couple of cm (from medium sizes onwards), and have the friction rubber outer layer. For me, it made a large difference, even though I will still place fig 8s and monkey fists as well, especially on routes where I have he placements dialled.
For example, in Bielatal there is one particular route of maybe 10m length. I normally protect the start with one very small monkey's fist made of 5mm cord. The next bit is a hand to fist size corner crack protectable by two knotted slings made of 11mm rope, and one absolutely bomb proof large monkey fist (made again from 11mm rope) that goes in just before the last section of 2.5m of ugly, flaring offwidth. This long was the last bit of pro, but now I can place a good, large UFO close to the top, protecting the beached whale topout.
CB