These ingeniously simple adaptors allow you to run two Cat 5 (or Cat 5e) connections over a single Cat 5 (or Cat 5e) cable. They are often used to save running an extra CAT5 cable when a port already exists and a second user port becomes needed.
A CAT5 (or Cat 5e) Ethernet connection uses only 2 pairs out of the 4 pairs available in the cable. Fitting one of these adaptors at each end of the cable allows the use of the unused pairs, enabling a second Ethernet connection to be made over the spare pairs, in the same cable.
These adaptors are used in pairs (supplied singly) - one fitted to the wall socket adjacent to the user, the other at the server room patch bay.
These are really handy devices to have, and have 'saved the bacon' of many a network administrator!