He hadn't been expecting that - later on, Giles will be able to admit to himself that he hadn't been expecting anything after getting ahold of the necklace. And so, as a result, just like Ginia, Giles falls to the ground with a heavy thud and a growl of pain as he feels her teeth dig into the thin skin of his wrist.

Nerves are a funny thing. No matter how much he wants to hold on, he'll let go after a few seconds of biting, and he's already having to focus hard just to hold on. Giles knows this instinctively. So, trusting to the fact that even she'll need a few seconds to get back upright, Giles makes a grab for Ginia's hair, intending to knock her head into the floor - just enough to daze, hopefully enough to daze, but you never really can tell in a bar fight.