Pâte fermentée is what we in the English-speaking world call “old dough”, even though it means something closer to “fermented” or “ripened” dough in French. Classically, the old dough technique is very simple. Every time a baker makes a batch of baguettes (or some other bread that might be improved by old dough) he or she simple reserves a piece of the unbaked dough and stashes it away for use the next day.
Over the course of the night some very interesting things happen. Yeasts continue to reproduce, creating alcohols (and as those of you who’ve read other posts of mine on flavor know, some flavor compounds are only “unlocked” by — i.e. will only dissolve in — alcohol). Bacteria grow and create flavorful acids. And enzymes proceed to run amok slicing and dicing long-chain carbohydrate molecules (starches) down into sugars.
All of this is a great boon to a baguette dough (or any other bread dough it’s added to), both in terms of flavor and color.