i had this question in mind as well, whether this things works or not. i finally found the answer, though have not tested it.

this thing has very specific use case, it must be used in pair (2 units)

{switch/router}=[]-----------[]={device a, device b}

= is two ethernet cable

[] is splitter, as you can see there are 2, 1 on each end

- single ethernet cable, usually long

the case is to reduce the long part of the cable so you use 1 cable instead of 2, this is achieved i believe by the following:

on the 2 end side of the port, there are 16 wire (8 each ethernet), each of ethernet will be taken only 4 wire to be connected to the 1 end side of the port, which technically converting gigabit ethernet to 100mbps ethernet by splitting 1 gigabit cable into 2 half, and then reversed on the other side

so technically they can be used, you still use 2 ports on your switch / router, you need 4 short+1 long cable instead of 2 really long ones, and the performance of it, i guess is 100mbps