Grab the bar as if you were going to do pulldowns. Inhale deeply and suck your stomach in hard. "Push" your abs against your spine.
Now blow all your air out as you crunch down. Contract hard and try to "cramp" your abdominals. Hold the bottom crunched position for a second or two and squeeze. It's a small movement, but do it right and your abs will be on fire. Do sets of 10-12 reps.
These are great for gaining control of your abs and for those who have abdominal distention.