Heres how i have gathered the detox drinks work: Whatever the fuck vitamins and minerals are in the drink cause your body to -stop burning fat- THC is stored in your fat cells, and gets into the bloodstream etc when fat etc is broken down and metabolized. By stopping that process, the drink stops the entrance of THC into the bloodstream for (i believe it says on the bottle) 4 hours, though i wouldnt risk trying your test int he 4th hour. The drink also makes you pee a ton, and along with some water, it flushes out your system so the THC that was already there is gotten rid of, and because your fat isnt breaking down no new THC is entering the stream.
I could, of course, be totally wrong, but this is what I was told when I bought a detox drink myself. I smoke every day as much as I can, and I passed with the detox drink twice.
Good luck man