Challenge your abs, hip flexors, and even the neck musculature with this combo exercise. Simply maintain the half sit-up position while performing flyes.
Anchor your feet and lean back until you feel your abs engage. You can use a lighter weight and do the exercise in a timed manner for 60 seconds, or use heavier weights and go for 10-15 reps. Keep your spine and neck as neutral as possible.
If you want to build huge muscles, you must continually challenge them by placing a greater demand upon them. If you stick with the same stress level month after month, you'll quickly reach a point where your body is used to the stress and won't need to adapt (i.e. grow) anymore.