It's simple. Fling the ball to Smash objects.
It's been a year since I last played FlingSmash under its original name Span Smasher. The Artoon-developed game is basically Wii MotionPlus pinball as you control a little ball on screen and send it careening through obstacles and enemies. If you want to know more about the basics of the game, check out our Aaron's impressions of what I assume was the same demo. For my E3 2010 impressions of FlingSmash, I'm going to focus on what's changed.
First off, this game was not a Wii MotionPlus exclusive last year. Now, it is, and that's definitely a good thing. FlingSmash benefits from the precision of MotionPlus as you have better control of the mayhem with the attachment. It seems to read more of the tilt and direction you thrust your Wii Remote in, and I'm not quite sure if a regular Wii Remote would work the same way.
The ability to stop your ball with the A button was added, which makes setting up your position a lot easier, especially when you get into tricky situations with bosses or puzzles.

They also added a MotionPlus compass at the bottom right that shows you how the controller is being held. Like the other additions, it helps make the game easier to play, which gives you the opportunity to play the game rather than fight it.
More styles of balls were shown this year. A metal ball, which weighs considerably more than the normal one, is sluggish to control, but more powerful when you get it going. There is also an underwater ball, but that wasn't demoed.
When it hits this summer, FlingSmash will have eight worlds with three levels and a boss fight each. An exact release date or price point isn't known, but if this game comes out as a budget title in the coming months, it would probably be a fun, worthwhile romp for MotionPlus owners.