Stand on an extender pad on one edge of the reformer (slidy platform on springs) and the other foot on the reformer platform. Push out the reformer so the platform slides away from the end of the machine and you are basically sort of doing the splits. Then, at the max of the stretch, arch your back forward, curving your arms together, so you are about bent in half. Start contracting your feet while slowly curving your body back into a standing position; then bend backwards so your sternum is pointing at the sky while opening your arms again. Close your arms and resume standing with one foot on the reformer platform and the other on an extender.
It's a great exercise for getting both kinds of motion in your back (forward bending and back bending) but let me tell you holding a bent forward position while you're pulling your legs together from half-splits is HARD WORK.
With that, it's bedtime.