The chantix bind themselves to the receptor, blocking the affects of the nicotine having any affect on the cells, and also cause you to produce about 60% of the dopamine that you get when you smoke. It takes around 12 weeks for your body to grow new cells that no longer have nicotine binding to them. This is why (i think) when you smoke while on the chantix it has no effect and taste like crap. When I was on it I would light a cigarette because of some cravings and have to chick it because it did absolutely nothing for my cravings anyway...As I understand it, Chantix works by blocking dopamine receptors to interfere with the "pleasure" you get from smoking. But those receptors are used for a lot more than pleasure.
That's my understanding of it anyway...