Well, crap... I told you wrong, gots my names mixed up. It's the thing where you elevate yourself vertically then put all your weight on your forearms and your feet are dangling. You then pull your knees up while letting your shins stay vertical, or lock your thighs and pull your entire legs up to horizontal.
A roman chair can be used for situps but it's most often used for your lower back, it's like a reverse situp.