It is a fan splitter, you run a cable from your fan port (aquaero, quadro, octo) to the 'IN' fan port on the spiltty4.
It works like any other splitter or 'Y' cable.
To control the RGB LEDs on the fans, you connect the RGBpx port on a quadro/octo to the RGBpx 'IN' port on the spilly4