Definitely flexing on the exhale, that's key.
I've been lucky to always have defined abs (even before I know that girls liked it!), so I can't really say how it helps with definition. I CAN say, however, that in my aikido classes, when I get punched in the stomach, I barely feel it. My teacher hit me full on, and I seriously didn't even blink - it was awesome.
Try for a couple days, you'll definitely notice a difference.