The kettlebell windmill is a great mobility exercise for runners. After more than 35 years of running, and not enough work on general conditioning, I found that my hip mobility was seriously compromised, and I'm not unusual among runners. Many runners, especially experienced runners, have very poor hip mobility. The lesson is that all runners should also take general conditioning very seriously. Don't just do token strength and mobility workouts, make them count!
Warning, if you try this exercise, do not be surprised if you cannot go through the full range of motion. If this is the case, do not try to force it.
Here are a couple of instructional videos to get you started.
Some Windmill Variations
Another Video on The Basic High Position Windmill