I assume it's a random genetic thing. Like a reverse-bending thumb. I can do that level of bend in all but my pinkey, while my thumb nearly goes 90 degrees.
It's actually kinda annoying, as my fingers sometimes get stuck in that position and I need to consciously unbend them or else they stay that way.
tbh something along the lines of what she did is a medical treatment for a certain STD; i forget the STD but they insert a mini-umbrella sorta thing in the penis, open it up, and scrape out the tissue