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.
Since I've started writing for the T-Nation, I've gotten 100's of e-mails asking me varying questions about the rotator cuff and its many somewhat magical qualities. While it may not be as cool as big quads or a great set of guns, a well-developed rotator cuff is sexy in its own, injury-free kind of way.