Cumming inside? I think
Alpha and omega au was originally introduced in "supernatural" fanfiction featuring werewolves. Basically knotting is a canid/canine trait or feature where the base of the penis balloons inside inside the other party to prevent the penis from leaving the orphis during ejaculation. Basically the penis gets stuck inside and can't be removed until the knot "deflates" and the erection goes down.
Knotting is the process which guarantees the pregnancy of an omega. It’s kinda hard to explain but that should give you an idea :)
It think some part of the penis enlarge while mating and it makes it impossible to pull it out without hurting the other one.
It lasts as long as the mating isn't finished and with canines, the male can ejaculate a few times before it shrinks enough to pull out.
What does knotting mean