It's probably best if you do drink all or most of it. I don't know if how quickly you drink it has any bearing on its effect. I made the assumption that, since it's basically an oral enema, and regular enemas go in all at once and quickly, that the swf should do the same.
What helped me get it down was this: Standing, I would take a deep breath and hold it while I drank as much of the swf as I could until I ran out of air. Then I'd hold my nose, or not breathe through my nose while I rested for a moment and then took another deep breath for another drink. I could usually get it all down this way in two or three drinks.
Sometimes, with a little of the drink left, my stomach would send signals that if one more drop came down, it was all going to come back up. So I would not drink the rest in that case.
Sometimes I would feel movement before I even consumed the entire drink.
Later on, I was able to get it down just by distracting myself from the taste by chugging it while I read or watched tv.