Lie flat on the inclined bench and secure your legs at the end of the bench (shoulder-width apart). Place your hands lightly on either side of your head, keeping your elbows out.
Begin to roll your upper back (scapula) from the bench. Rest your lower back on the bench. While contracting your abdominal muscles, your scapula should come up off the bench completely. As you breathe in, go back down in the same way. Breathe out as you go up. Repeat as required. Keep your elbows out and maintain scapular contraction.
Along with correct breathing, maintain a neutral pelvis position to avoid overarching your lower back (lordosis).
Concentrate on abdominal breathing!
Do not jerk and focus on correct abdominal contraction.
To eliminate overarching your lower back, put your pelvis in a neutral position and push the small of your back down into the floor.
This is a variation of crunches: lying on decline platform makes the exercise harder and it is also possible to lift the body higher thus making use of longer trajectory.
Hands can be either on the side of the head (touching the ears) or above the trunk, especially if you choose to hold a plate in stretched hands.
Holding a plate (or other weight) behind the head or locking the fingers behind the head can cause lower back problems as the head is being pushed forward.
Notice: there is a difference between crunches and sit-ups. In sit-ups you lift your upper body completely into or very close to sitting position. This movement is mainly strengthening internal muscle called psoas, not the external visible abdominal muscles.