She could break his grip if she wanted to, easy as a lie, but this is new and fascinating, and if she's being entirely honest, Mina's more than a little aroused by this side of him. And finding herself in this position again reminds her of someone else entirely, so she's content (mostly) to leave the shadow of the vampire behind them.

"Why?" she asks. "Why would I stop when the reaction is so good?"

She thinks she can smell the return of his arousal under the fury, the possessiveness and it just makes her grin.