Copulatory (coital) tie
Dogs Copulating, by Harald Groven
The swelling of the bulbus glandis, once inside the female reproductive tract, causes what is known as the copulatory tie. This is sometimes also called the coital tie, or just tie.
After intromission, the male will usually step down from the mounting position so that the two animals are in the position you see here.
During the tie, the two animals are physically unable to separate. The copulatory tie usually lasts anywhere from five minutes to an hour, and after this time the animals will separate naturally. It is important that humans to allow this natural separation to occur. Major damage can be done to the reproductive tracts when humans try to physically separate the animals prematurely.
It's not unusual for dogs to mate and tie several times.