Get in the half-sit-up position with your feet anchored. Maintaining this half sit-up position while doing any movement with the arms will challenge the abs, and this one hits the delts as well.
Try to keep your spine and neck as neutral as possible. Go light and do the exercise for one continuous minute, or go a bit heavier and aim for 10-15 reps.
When it comes to exercise performance, especially the more exotic movements I often prescribe, most people grasp the big picture but miss the finer points. To remedy this, I created this "toolbox" series to help experienced lifters fill in the blanks and newer lifters learn about some very effective exercises. Here's the newest installment!