This old tick lets you have TWO Ethernet connection on one cable. This is for a normal 100 MHz LAN. Of the 8 wires, only four are needed to make a connection. So yu can use the other four to run another LAN connection over one cable. This is HARDY when you do not have a HUB to tie two PCs onto a LAN. Or, you can have two independent LAN connections.
You can find videos on Your Tube  shoring how to use the spare wires.
Here is one:
https://www.youtube.com/watch?v=ZGvhW-qOqAk
How does this work?  Read this:
https://superuser.com/questions/764576/why-does-just-splitting-an-ethernet-cable-not-work
It  is remarkable that so many people said it can not be done.  It *works* in the sense that you can get some communication. However, it does heavily rely on the collision detection mechanisms of ethernet and despite that the systems will also get quite a lot of network errors.I can't see any benefit from doin it this way when there are simple solutions...just me. Quote from: patio on October 18, 2019, 06:32:56 PM

I can't see any benefit from doin' it this way when there are simple solutions...just me.

've purchased several of these kits and actually used the 'method' before by creating the splitter myself. They work very well. This kit takes advantage of the fact that most network CABLES have two extra pairs of wires. It splits the 4 pairs in the cable into two sets of two pairs to create two network connections over one 4-pair cable. You do need an extra port on your switch or router for the second connection. If that is not available, you need to buy a small 4 or 5 port switch instead and put it where you need more connections.