The partner bench back extensionis a clever way to perform this exercise, which works the spinal erectors in the lower back. There are machines and angled apparatus that allow this exercise to performed solo; but, lacking those, all you need is a bench and a workout partner.
Works the spinal erectors specifically and core more generally
Lie prone on a flat bench with your shoulders and head off the bench. Your midsection and legs should be supported and your partner should firmly press down on your upper hamstrings to keep you in position on the bench.
Once secured, lift your hand off the floor and cross them over your chest. This will be your starting position.
Contract your lower back to rise off the bench as high as you can, then slowly release and extend down as far as the range of motion will allow.