In their setup, the Din cable is used for the balanced Symbilink signal (it replaces the RCA's as the amp's input signal cable). Basically the Din cable has 6 (IIRC) pins in it; 1 pin for L+, 1 pin for L-, 1 pin for R+, 1 pin for R-, 1 pin for 15V+ and 1 pin for 15V-, then the outer sheild is the ground for all signal carrying pins.
In this type of balanced system, the source for the signal (probably Zapco's Symbilink transmitter if you are going to use Symbilink) sends two signals for the Left input signal (one positive and one negative) and two signals for the Right input signal (one positive and one negative). The positive signal is the exact same signal that the headunit outputs from it's preouts, and the negative signal is the exact same signal, only inverted (out of phase with the original signal). So when the amp receives these two signals, obviously they are going to be equal but opposite and cancel each other out.......which means that any "noise" that the amp is still recieving after those two signals are cancelled out must be noise that isn't supposed to be there //content.invisioncic.com/y282845/emoticons/idea.gif.5acb6a39a9b92425414c316dda202bad.gif So you now have a perfectly noise-free system (or very, very close to it).
You can use it with any headunit, as long as you buy the necessary Zapco transmitting equipment.
I'm really bad at explaining stuff, so I hope that makes some sort of sense........