Yes, the knot goes inside the vagina and locks the female, it's called "butt-to-butt stage" or "the second stage coitus" --
If the knot is big enough (bigger dogs) it stretches the entire female-human clitoris area which can produce immense pleasure, also the dick can move around inside while it's locked (3rd figure). Also remember the second stage coitus can last as long as 45 mins, where a average healthy human-male can go at most total of 20 mins without any drug (humans don't have second stage, just the first stage).
You can't just beat the dog boyo, even a 9" BBC can't go anywhere close to a dog-sex, there is no fucking way.