I always believed it was the burning of the THC that got you high. I didn't think about the other persons in the rotation not getting stoned because you burnt up all the THC the first go-around. Maybe there is a technicality that ensures everyone gets stoned from the same bowl? Since the flame is pulled through the bowl-hole it does not actually burn ALL the THC the same way it would if someone were to torch the whole bowl. And also, you do not keep the flame to the bowl. You light it once, just enough to ignite the weed partially, then the next in-line gets to do the same thing and still achieve the desired results.