Perhaps it's that he managed to take away so much of her resistance to his orders long before he collared her and chained her to his will so tightly. So tightly that she can't even imagine doing anything but following that command in her mind, milking his cock with her actions.

She whines for him, a thin needy sound, wanting so badly she can't do anything else. It feels so good, and yet...she yearns for more, no longer even certain whether that more is her own release or to be further filled or something she hasn't yet imagined.