פירוש על קידושין 49:19

The Talmud answers that we do not require that the water actually enter the mouth and hit all the teeth and the tongue. But there cannot be anything preventing it from doing so, had the person opened his mouth.
This is compared with the case of mixing flour and oil for a grain offering. If the flour is fit to be mixed, then the mixture if valid even if it was not actually mixed. But if the flour is not fit to be mixed, then mixing becomes indispensable. In other words, you don’t actually have to mix the flour. You just have to be able to mix the flour. So too the tongue need not be immersed, it just needs to be able to be immersed.