Lie down on an incline bench and secure your legs at the end. Your knees and feet are shoulder-width apart. Bend your knees to a 90-degree angle. Put one hand beside your head just above your ear with elbow pointing outwards. Your other hand rests at your waist.
Raise your upper body slowly while turning your torso towards the opposite knee. Rotate your torso with the hand beside your head towards the other hand resting at your waist. Continue crunching until your shoulder blades are completely off the bench (seated position) as you contract both transverse and oblique abdominal muscles. Breathe in and go down in the same way. Breathe out as you go up (to the seated position). Repeat as many times as required. Keep your elbow pointed outwards, your shoulder blades in. Exchange hand positions and switch sides with an equal number of repetitions.
Make sure to maintain the correct position of your feet and a sufficient knee angle. Along with correct breathing, make sure to maintain a neutral pelvis position while avoiding lordotic back curvature (straining your lower back). Focus on abdominal breathing! Focus on keeping your abs tight and keeping the movement slow and controlled; avoid jerking.
Make sure to use the correct technique and muscle contraction for the best result.
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, locking the fingers behind the head can lead to upper back problems as the head is being pushed forward in an unnatural position.
In oblique crunch it is not possible to add weight so slow motion and concentration are important.
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 external visible abdominal muscles.