Acid

An enzyme in flour called protease converts some of the insoluble proteins in the flour into a soluble form, mellowing and softening the gluten strands and making the dough easier to roll. This requires time and involves a long rest period.

Adding acid performs a similar function to the natural enzyme in the flour, but it does so more quickly. Some bakers add 1 teaspoon of lemon juice or vinegar to the liquid when forming the dough package for puff pastry. Cream of tartar is an acid, but I prefer the flavor of a liquid acid in my doughs; its liquid form also makes it easier to distribute in the dough. Sour cream, crème fraîche or the milky residue from clarified butter are wonderful liquid additions to doughs, making them more supple to work with.