Set up a glute-ham bench or similar to place the pad under your upper hamstrings. Lean back under control as far as you’re able, and then crunch the abs and sit back up, tightening the quads as you rise. Tight glutes in the bottom—and a properly positioned pad—will help prevent the hyperextension of the lower back that commonly causes pain in this exercise.
 
You can also limit the range of motion if the full movement bothers your lower back, or as a way to make it somewhat more difficult, by stopping with your upper body approximately horizontal.
 
Notes
This can be an extremely taxing exercise for athletes who have never performed them previously—ease into them with a very low number of reps and sets (e.g. 3 x 5) the first exposure to gauge your response.
 
Purpose
The GHD sit-up provides a much longer range of motion for the back and hips than a traditional sit-up and will add a stretch of the abs to encourage more strengthening.
 
Programming
GHD sit-ups  should be done at the end of a workout and can be done for sets of anywhere from 10-50+ depending on conditioning.
 
Variations
You can pause in the horizontal position, use slower tempos, add rotation, and add weight. Without weight, the arm position can be changed to increase or decrease difficulty—holding the arms at your sides, hands on the chest, hands behind the head, and arms extended overhead will make the movement progressively more difficult.