Dogs get impregnated when they "tie". The sires penis ejaculates with this gooey stuff (Semen or prostatic fluid) along the the entire member...it does
not squirt out like humans do. The dams vulva swells and holds on to his penis, like a suction. That's why they say not to pull them apart or pour water on them to untie them, because it can damage them. When the male is done ejaculating, his penis will go back to it's regualr size and the dam's vulva will release. This normally takes about 10 minutes or so.
Here is a website with more info, but you can do a google and read up on more stuff.
Breeding - Tie, Male and Female dog breeding tie