The decline sit-up is a trunk and hip flexion exercise that allows greater difficulty through a longer range of motion compared to the sit-up.
Secure your feet in the pads of a decline sit-up bench. Hold your fingers at the sides of your head or across your chest. Crunch the trunk and sit up to bring yourself up as far as possible. Return to the bottom while maintaining tension in the abs and repeat.
The decline sit-up provides trunk and hip flexion training with resistance farther along in the hip flexion movement relative to a sit-up on the floor.
Unweighted decline sit-ups should be done for sets of 10-30+ reps; weighted decline sit-ups can be done for sets of 8-15 reps.
Decline sit-ups can be done with additional weight (such as a dumbbell or plate held behind the head/neck) or with rotation, and of course with the sit-up board at different angles.