Worm Burner... that's what it used to be called.
Video by Sofi
I got slammed a great many times wondering the same thing. I'm so glad Sofy told me to pull even harder.
The first time I ever saw this trick was in Windsurfing Magazine. I think it was a sequence with Mitch Gingritch, and I looked at it and wondered, "how the hell do you pull the clew through the wind without getting slammed?"