Hold your brush like a screwdriver. It's an awkward way to hold a brush at first, especially for painting watercolors.
I've used this grip in acrylic paintings and oil paintings. Its use in watercolor can serve several purposes.
First, the limited range of control forces simplicity. Secondly, holding your watercolor brush in this manner gives access to broad "sketching" or "painterly" strokes. Thirdly, if your fingers shake or you have arthritis, you might consider altering your style by altering the way you work your tools.