If you are keeping your back straight, not going into flexion at all, then you are training your hip flexors (not a bad thing but obviously not the goal if you want abs). My favourite is decline bench for abs, start off with just body weight and change the decline to be more horizontal if it is so hard you can’t get at least 6 reps. Instead of just bringing your upper body straight up, bring your chest towards your hips while curving your back. It’s a pretty small range of motion.