Here's an excerpt from Tom Lehman (the "Dough Doctor"):http://www.pmq.com/lehmann_winter-97-98.shtml
There are a number of different types of "conditioners" which can be used in making pizza dough, but for this article we will limit our discussion to reducing agents, the most commonly encountered type of conditioner used in pizza dough production at the pizzeria. These are products, which by nature of their effect upon the flour proteins result in a softer, more extensible dough. The active ingredient of these products will usually be one of the following: L-cysteine, sulfate, or glutathione (dead yeast).
As I've mentioned previously, these products can be used in making short time doughs, but where they really shine is in helping to achieve a slightly modified dough, or finished crust characteristic. As an example, if your doughs are exhibiting a little too much shrinkage after forming, the addition of a small amount of one of these products to your formula can correct the problem without having to make any other changes to the formula or procedure. Will the use of these products be perceived in any way by my customers? No, not if they're used correctly. Again, using L-cysteine as an example, when products containing this material are used within the manufacturer's recommendations, the effects are very predictable upon the dough handling properties, and there is no flavor or mouth-feel issue in the finished crust. However, at higher levels, beyond the manufacture's recommendations, this material can impart a slight burning sensation to the lips and leave the customer with a sensation of being thirsty.