
Hi Sarah!
When dogs mate, it starts with the dog mounting and penetrating the bitch as you'd expect. The dog has a 'bulb' in his penis which expands with excitement, and when he's in the bitch this swell, and her muscles tighten on it, and they are physically locked together. It is impossible to separate them at this stage without seriously injuring one or both of them. This is when the dog usually 'turns'. He moves one of his front legs over the bitch's back, so that both front feet are on the same side of her, and on the ground. Occasionally he'll stay like this, but more often he will then lift a hind leg over her back as well so that they are standing facing away from each other, still 'tied'. This lasts anything from a couple of minutes to over an hour, and is when both animals should be controlled so that they don't hurt each other.
The 'tie' ends naturally when the bitch's muscles relax and release the dog, who needs to be checked to make sure his penis has shrunk back to naormal and retacted within its sheath.
Hope that explains it.
:)