Yeah, acid will remove some stains, by reacting with the stain, and turning it into something that doesn't "grip" fabric as tightly. Same idea as bleach really, bleach oxidizes a stain changing it to "something else" which is "not stain" The trick is to use a weak enough reactant that won't damage fabric.
Or is the question what happens when you mix cream of tartar and vinegar? Well, what is cream of tartar -- creamy tartars? Or something else? (I know you know it's chemical name, but what does that name mean to you?)