In typical interweb fashion, I'll explain even though I have never used them.

Take a look at how your legs move while spinning the cranks. One leg is doing all of the work while it is in the down stroke, while the other leg is very limited (muscle exertion) in the up stroke. By fixing your foot in place to the pedal, you will be able to use your muscles on the upstroke as well. Reportedly makes it more efficient.

But like I said, I have no direct experience with them, nor do I plan on using them, until I get a recumbent trike.