The reverse hyper is one of the best pieces of equipment around for strengthening the posterior chain: the hamstrings, glutes and lower back.
Westside's Louie Simmons invented it, so you'll see it a lot in hardcore powerlifting gyms. And today many CrossFit boxes are adding them. There are several ways to use the machine – unilaterally, bilaterally, swinging or strict – but in the video here we'll just go over the basic function.
Do as many good reps as you can in the 4-6 rep range. Your goal will then be to double that number of reps. To do that you take short rest periods (15 seconds).