If you were to go to you local sail loft for a cat, it would be laying there perfectly flat. The batten pockets also would all be flat.
The luff of the sail would also be flat but from top to bottom there would be an arc. So, When you put that flat sail on a straight mast that extra arced material is the fullness and makes the battens bow out -- for a full sail, great for lighyt air.
As the wind picks up and the boat flies a hull, you first hike, then trapeze, keeping the powered up sail.
Now more wind. And now, despite your trapezed ballast the boat is still trying to fly a hull, apply the downhaul, (which bends the mast thus absorbing that arcing material) a little bit until it becomes controlled healing. If the air lightens a bit and you see the windward hull is no long just out of the water, ease off the downhaul. That straightens the mast again, getting you more fullness and more power.
If the wind continues to build, apply more downhaul until under control again.
etc., etc., etc. until you run out of downhaul. Then you might have to let the traveler down an inch or so at a time.

Best to have something like 16:1 or 18:1 downhaul. Not that it takes that much to sheet it in, but with those ratios when the air lightens up you can flip the control line out of the cleat. Lower ratios and you can't, meaning the crew has to get back into the boat to ease the downhaul.

On the Laser the most important tool is the vang. It does the close to the same thing -- flattens out the mainsail.
Rick