We bought many arcs for our Pilates classes. It has been an amazing tool to share with a class. I have discovered in my own practice and teaching others how the arc opens the spine and trains it for dynamic movement.
When the spine is tight, one will try to initiate the movement of the spine such as in the crunch and Pilates rollup from the neck and/or shoulders. The beauty of the arc gives information where the spine is tight and not sequencing.
I start out slowly. I only allow the client to roll back to where the spine can easily extends. I repeat many times the rollup with each time trying to move the spine more into extension. I make the client aware where they are initiating the movement; the spine or bad version from the neck, shoulders or the back. My goal is to get them to comfortably extend the spine. I might use props for the head if the spine is not ready to extend comfortably.