Whip her.

You take the whip out of the bag and whip it through the air so it makes a satisfying crack. Rosalind shudders at the sound, but doesn't try to protest.You're a little nervous about this. You've certainly never whipped anyone before, but you've been really mad at her for a long time. You decide you'll be careful and probably go easier on her than you want to, since you really don't want to make a mistake and do any serious damage. Before proceeding, you admire her reddened ass from the paddling you administered earlier. You stand behind her and swing the whip down so it cracks onto her buttocks. Her head jerks up and her eyes widen. She gives a loud grunt through her gag. You look carefully at the red stripe that now crosses her ass, standing out even against the general redness from the paddle, and nod to yourself with satisfaction. There's no question she's going to feel this every time she sits down for at least a few days.

You proceed to beat her rather mercilessly, though not as mercilessly as you think she deserves, until she's crying and whimpering. At no point does she try to move from her position though, so you conclude she's been well trained by somebody.