Flat rockered boards usually get going early and are fast: Alaia, Skim, Door. Some say, well, concave works because you have less rocker along the center line. This is partly true: less rocker usually makes for a faster board. But it's not completely true.
If you make two boards, equal outline, equal flat-ish rocker along the center line (stringer). One board has no concave. The other board, with the same rocker line along the stringer, has got concave, which means it has more rocker along the rails. Which one is faster? If you shape it right, it's the board with the concave. Not the flat board. I tried it.
Then - there are exceptions from the flat rocker rule. Fast boards with lots of rocker. Like the OR Mako for example.
And there are boards with single concave, which are complete dogs. I won't point any fingers here.
Now - how does that all make sense?
Here's how: the trick is a single concave, WELL SHAPED IN RELATION TO THE ROCKER. It's the saddle shape. A fast shape is when you achieve a saddle shape with a straight line PARALLEL TO THE WATERLINE.
Let me explain:
It's not the line along the stringer - the rocker - which has to be straight. It is the waterline, in the board position where we usually want to go fast: upwind. Going upwind, our board is pointing slightly higher than our course through the water. The red line in this kid's pic (stolen from the web, thanks Drew!) is the water surface line. This is the line we are talking about. This is the one which has to be straight, to make for a fast board.
Now how would we achieve this in a surf board? Here are some pics from the shaping process.
-The rocker is not straight, it's a curve.
-Concave is also a curve.
-But: rocker + concave form a saddle; if you shape it right, this allows for a straight diagonal line.
That's just mathematics

I always check the bottom shape with a long ruler when shaping. Look a those pics:
-Rocker is curved;
-board has concave;
-resulting diagonal line is straight.
ROCKER CURVE:
CONCAVE all along the board:
THE RESULTING DIAGONAL LINE: STRAIGHT (not perfectly, but almost; this is not the final shape. final shape is perfectly straight)
This is the one most important thing I learnt about shaping a kite board.
There are other things like the outside curve, sharp edges, fin toe in, which also play a role. But first comes, by far and large, this little detail of the bottom shape. Second (for a surf board) would be fin toe in.
Now I've spilled the beans for you

cheers, tungsten