This is one of my all time favourite exercises to do and to teach!
It has many different variations, all of which work you in slightly different ways.
In the one pictured we move the arms away as the spine moves up off the bench into a spine curl.
I love that it works on scapular stability and awareness, rib cage alignment, spinal movement and control. It also gives you a gorgeous oppositional stretch, think of your hands and arms stretching out one way and the knees lengthening the other way.