FOR THOSE OF YOU WHO DON'T KNOW WHAT IS GOING ON:
Basically when the male dog done doing his business with the bitch, the bulbus glandis of the male's penis swells up inside her bajingo. It's called a tie which can last anywhere from 2 to 30 minutes, but it usually lasts from 15-20, and during this time he releases prostatic fluid.
As soon as the dogs lock the male usually moves his leg over her back, turns butt to butt and they remain locked. Tied.
also this picture is pretty cute:
Swelled bulbus glandis: