A concave bottom redirects the water flow downward.
It generates lift, gets you up on a plane with less power, makes the board feel more "grippy" and lets you edge against the kite with the board slightly flatter.
Contrary to what you might think, the water flow under your board does not move from the tip towards the tail.
In fact, water flow is divided along a line close to the centerline of the board and moves backwards but at an angle toward the rails. It's easy to prove this: just look at the rooser tail comming off of your toeside edge when you are edging your board against the kite. You can't see it, but the same thing is happening on your heelside rail.
If you are looking for an inexpensive board with excellent performance in marginal winds, I build a epoxy/ laminated birch board that measures 150x46 with a concave bottom and very fast rocker line. My construction techniques create a board that is stronger, stiffer, and lighter than a home-made plywood board. $330 US plus shipping for a complete board including NSI micro-adjustable footstraps.
pictures of a few of my boards are posted at: http://gallery.kiteforum.com/gallery/vi ... me=albup46