This is an exercise for the lower back that is simple to perform and can be done almost anywhere and requires no equipment. Your lower back, Erector Spinae, supports your upper body and affects almost every activity that you do. It is important that you exercise this part of the body.
The lower back extension is probably the best exercise for strengthening your lower back as well as eliminating lower back pain, loosening tight or sore back muscles that may be caused by sitting on a chair all day.
This exercise is also recommended by chiropractors and doctors. If you do suffer form lower back pain it would be advisable that you check with your doctor or chiropractor before attempting this or any exercise.
I've also seen variations of this exercise done in Pilates and Yoga.
This exercise can be done on vertical benches or holding onto extra free weights. The exercise done on a vertical bench is much more difficult. Personally, I do not feel that it is necessary to perform this exercise on a vertical bench since doing it on the floor is just as effective. Using a vertical bench may also put undue strain on your back.