The CAT Y-adapter allowed just over one CAT cable to run two Ethernet cables (Cable-sharing). This is possible, since half of the wires is not used in the cable network. The CAT Y-adapte does not use electricity, but must always be used in pairs - one at each end of the line. The first piece combines the signals on one cable, the second piece separates them back to normal network sockets. However this only works if you use no gigabit LAN and no PoE. There namely more wires are needed and the trick does not work.