For use with any SportCrafters Trike or Handcycle Trainer. Allows the trainer to move fore and aft slightly to offset any “rocking” motion caused by pedal stroke. Not an essential accessory for all trainer users, but has been found especially helpful for handcyclists.  The motion of the trainer in the Rhythm is random, and not annoying at all.  In fact, here's how a customer describes the Rhythm.. which is the best illustration I've ever heard:  

Pete;  I wanted to let you know how pleased I am with the Rhythm.  I found the minimal fore and aft motion actually rather pleasant.  It reminded me of a couple of activities that took place several thousand miles apart, and many years ago.  I was born in northwestern Canada in the early 1900's, and horses were the main mode of transportation and the gentle rocking of my trike reminded me of the gentle gait of a well trained horse I often rode.  The other reminder took place in the the Bronx, NY in 1959-60.  I was in college and on weekends would take the train to New England and the gentle rocking of the train was very restful.