Just the lauryl/laureth sulfates but who cares you're leaving it on your scalp for about 5-10 seconds in a pre-wash anyways. That's all it takes for a pinch of shampoo in the next wash to lather up really good. Only the surface area of your scalp can absorb anyways so I see no point in using more than will cover your scalp in the time you leave it on there. If your head is still all sudsy after you rinse than it seems like a big waste of the good/expensive shampoo IMHO. You want the conditioner to sit in your hair though afterwards.
washing your hair with a shampoo and rinsing it out before your special shampoo in order to get the hair lathered up. this way you don't have to use as much of the expensive shampoo and it lasts longer. you can use a pea sized amount instead of a dime or a quarter and your bottles will last 6-8 months instead of only 2 or whatever. using a cheap prewash shampoo like suave or whatever will save a lot of money.