It's kind of hard to imagine how the swing works, but if you've ever noticed that it's hard for one of your knees to bend as far as the other one or you've noticed that one knee has a hard time straightening out all the way, this is for you. This allows you to maintain a "normal" pedal stroke on your "good" leg, and a modified pedal stroke that will be more gentle for your other leg.
Pedal not included; Does not work with Morse Taper Pedals