Wind Turbine Blades

On another forum I visit these was some discussion regarding the design of wind turbines - the number and size of blades, various configurations etc. A question I had that no one answered had to do with what I can only call a "dip" in the blade out near the tip. I looked for a good image but couldn't find any that show it well. I don't know if all/most blades are designed like this but the ones where I live are and it is obvious if you are close enough. Tip vortex reduction?