Anchored on a decline bench, hold your fingers at the sides of your head. Crunch the trunk maximally to push the lower back into the bench before hinging at the hip to sit up as far as possible. Return to the bottom while maintaining tension in the abs and repeat.
 
Purpose
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. Having the feet anchored also makes it easier to maintain tension in the abs throughout a set.
 
Programming
Unweighted decline sit-ups should be done for sets of 10-30+ reps; weighted decline sit-ups can be done for sets of 8-20 reps.
 
Variations
Decline sit-ups can be done with additional weight (such as a dumbbell or plate held behind the head/neck), with rotation, with slower tempos, with pauses, and with the sit-up board at different angles.