Nothing, try to run

You look up at her and you know she isn't messing around. Either you run or you lose your manhood willingly, at least one of those options has a chance of keeping your balls. It's a no brainer.

After looking her in the eyes you look at the empty street behind her, feigning surprise. You mutter just loud enough for her to hear but quiet enough for her to believe you didn't mean her to, "The fuck?"

As soon as she glances over her shoulder you stumble to your feet and start running, however given the knee you had taken just seconds prior your speed is much slower than it would otherwise be.

After not very long at all you hear footsteps from behind you and look down just in time to see her foot plant itself firmly in your crotch from behind. You fall forwards onto the ground due to your momentum but manage to save yourself from further injury.

You hear her laughing to herself about how silly that was from behind you.