This exercise is typically done with ropes and a cable pulley system, but I like doing them with a band.
Banded Decline Sit-Up with Eccentric Emphasis
First, this version is much more convenient than hauling over a decline bench to your pulley system.
Secondly, using a band instead of a pulley changes the resistance profile. In English, that means that near the bottom of the rep (near the top of the bench), the resistance is little to none. At the top of the rep – where the hips are the most flexed – the resistance is at its max (since the bands are the most stretched).
This is a great modification to safeguard long-term health, especially for people who have a history of lower back issues. The key is to accentuate the negative rep and descend as slowly as possible.
Take a close look at the movement pattern. Don't underestimate this. Just like the decline rollout variation, increasing the size of the lever arm (by reaching above your head) makes a world of difference. It doesn't take much weight – and definitely not a thick band – to get all the benefits. Anything more and you'll lose your form in a hurry.