The KNOT is a Slang term used to describe the Swollen area of a Canines penis. This swollen area is call the "Bulbus Glandis". Its is located towards the base of the penis. During the act of mating, when the male dog enters the vagina of the female dog, this area swells up. Once swollen, the dogs will be "TIED" together for upto 25min while the male is ejaculating. This increases the changes of conception by sealing the semen in the uterus. Since the Females Vagina points downward, the semen would simply drain out due to gravity.
This you see the size of the Knot on that dog?