Baking ammonia is the chemical ammonium carbonate. It decomposes during baking to form carbon dioxide gas and ammonia gas. Only heat and moisture are necessary for it to work. No acids are needed.
Because it decomposes completely, it leaves no residue that can affect flavor. However, it can be used only in small products, like cookies, which allow the ammonia gas to be completely driven off.
Baking ammonia releases gases quickly, so it is sometimes used in products like cream puffs where rapid leavening is desired.