[Donut]

IFI has most of the documentation you need for the OI I believe:

http://www.ifirobotics.com/oi.shtml

The reason for being able to do this is basically user convenience; if I hook up joysticks to ports 1 & 3, but need to use more inputs for my control system than is available from just 2 & 4, I can "steal" the inputs from 1 & 3. This saves me the hassle of having to build a special connector in between the joystick and competition port on ports 1 or 3 so that I could use their inputs.

Our team has done this numerous times, it can really be quite useful (especially when you're using 13 of your 16 digital inputs).