What a DJ Splitter does is send one side of the output (EG LEFT) to one 3.5mm socket and the other (EG RIGHT) to another 3.5mm socket. This lets you have a separate headphone output to your main speaker output from one 3.5mm jack, although both of course will be in mono. Below is a wiring diagram that should hopefully make sense.
The top 3.5mm jack is from your laptop/tablet, the bottom left one has all the left signal and the bottom right one has all the right signal.
Blue = left signal
Red = Right signal
Green = Ground Signal
Note how the left is wired to 2 pins on the bottom left jack and the same for the right – this means that the signal comes out of both headphones.
Normally the bottom 2 would be socket’s but I couldn’t find a decent diagram of a 3.5mm socket online.. so you will have to imagine it it one.. 🙂

bob6397