Arrived promptly, it's got an excellent diagram of the wiring, it does exactly what was described, very useful, an absolute bargain.
Note: it allows you to run two Ethernet devices down 1 cable. It can't split a router port into 2.
Now where do I get something very similar which allows me to split out cables 4, 5 which carry voice, while 1, 2, 3, 6 carry the ethernet traffic :)