Just went over the AGP 3.0 specs in the other thread. It has a X8 transfer ability. The specs are both Intel so the underlying techniques are possibly the same.
In the AGP x8 scheme, it is basically simple. They make the data lines very busy. Eight bits per 66mhz AGP period. A date line would look like a 233mhz square wave if the data alternated 1's and 0's. That will get you 466 megabits / second on each line. To reduce the power consumption of such a fast buss, they also reduce the voltage swing to 0 / 0.8 volts for the data states.
If the scheme is the same with the "quad pumped" 100mhz buss, it would get you 200mhz square waves with alternating 1's and 0's and 400 mbits per second per line.
Easier said than done, but in principle, just binary coding.