Thanks for the compliments on the shots Matt, and Bambi.
Andrew, you are correct about the swelling part, but, the female can actually become impregnated by several different dogs from subsequent breedings, no matter if she has already been successfully bred prior. You can actually have two to three different fathers of the same litter if you are not careful.
The mating tie ensures that the sperm will get inside the vagina and that the female dog will be impregnated. Since the dogs are stuck together the male will be able to finish releasing his sperm. The mating tie prevents sperm leakage.
When we breed our Cao de Fila de Sao Miguels they tie for 30-35 minutes, every time.