Buffy holds her wrists together, as Spike snaps the cuffs around them. He leads her to his bed and attaches the cuffs to the headboard and expertly spreads her legs wide, her ankles securely chained. Even slayer strength can't help her get any leverage.
Her heartbeat is steady, there is no fear or anticipation. Until he finally thrusts hard, she makes no sound. She cries out then, turning her head to the side, exposing the scar on her neck.
Spike grits his teeth and looks away. He knows she allows this not because she trusts him but because she doesn't.