Your teeth get whiter. Why is that? Because the oil removes bacteria which causes plaque and tartar.
I don't now why OP is more effective than traditional alcohol washes. A good friend of my Dad's, a noted biochemist, thinks its the acids in the oils that do that trick (lineoic, oleic, etc).
I hesitate to say this, but Bruce Fife wrote a book about Oil Pulling and it does give sound supporting information on why this works. It does have to do with the bacterial load in your mouth. It's the reason you have to take Antibiotics before a dentist will perform gum surgery....same idea.
The reason I hesitate to tell you about Fife's books is that with some research, you can find everything you need to know on this forum for free. Fife actually lifted a bunch of stuff from us and from Earthlinks forum.