Anchor your feet under something and lean back until you feel your abs turn on. Try to keep your spine and neck as neutral as you can. You can go light and do the exercise for 60 seconds, or use heavier weights and shoot for 12-15 reps.
Maintaining the half sit-up position while performing any movement with the arms will challenge the abs, hip flexors, and even the neck muscles.
This article contains so many training concepts and ideas that it'll likely keep your workouts fresh and productive for months to come. This is one you'll want to print out and save.