Kettlebell swings are a great exercise. They have several different places in my programming, ranging from teaching a dynamic hip hinge on the way towards Olympic lifting, as a replacement for Olympic lifts because of an injury, as a "power endurance" tool, or a late season- low load/high velocity power exercise.
A good friend of mine, Jack McCormick from San Diego recently showed me this variation of the swing. By incorporating a band to the exercise, we can effectively increase the weight of the bell, which is helpful if you only have kettlebells of a certain weight.
Another purpose of the band is a concept known as "accommodating resistance". Similar to benching or squatting using chains or bands, the Kettlebell Band Swing allows for more emphasis on acceleration because the band acts like a breaking mechanism at the top of the swing. Basically you can extend the hips as powerfully as possible and not have to worry about decelerating the bell at the top, since the band pulls back as the bell moves up. It also speeds up the return of the bell during the swing, so it adds an eccentric overload which is hard to get any other way.
I really like this addition to the already great exercise, the kettlebell swing. I feel it has added a component of power to our program which is very beneficial.