We are not loading the fins like on a windsurf board on a kite race board. Yes we are maxed out on pushing the fins, but it is a fine line not to put to much pressure on the fins. A lot of the pressure is in the harness.
Keep in mind we do not have a mast and rig loading the board through a universal joint. We do not make the fins go through the water with as high of an angle of attack as a windsurf (as on a formula windsurfer) We need 3 fins to keep the board railed up. If the windward rail goes down, then you tend to spin out or get thrown over the front of the board.
Its about the fine line of keeping just the right amount of load on the fins. Thats why you have to ride a race board with a custom adjustable trim system and you have to adjust the depower for almost every puff, which you can see of the better videos of the boys riding here.
Watch the video starting at the 1:10 minute mark and you can see that Adam is always holding the trim line and using it to keep his body constantly hiked out. This is the main point that people just getting into kiteracing do not understand. You are not loading the board the same as riding a twin tip. A race board does not flex.
The race fins flex a little, but not much. The Body position of hiking out and counterbalancing against just the right amount of load against the fins is the trick, and the tricky part. If you are riding a race board and someone is riding a twin tip, you would be one or 2 kite sizes smaller on the race board, as you rely on the power of the fins as much as the power of the kite. Its a fine tuned balance game. http://vimeo.com/25127199
If a puff hits and you have to sheet out and get pulled up over the board, you are not loading the fins anymore and lose control of the board. The exact proper "Hiked out to the max" body position has to be kept all the times, which you can see in the videos.
There is flex in the Rista fins but not nearly as much as say a formula fin. Thats why Rista is making the best fins, tons of testing, in template outline, proper rake, proper cant, flex thickness and the foil shape....he's super into the details (hey Paolo I need some speed fins for Rob, really!!)
By the way Paolo "Really" Rista was once one of the top 10 Pro PWA windsurfers in the world and has a background in Marine applications of engineering, he's a pretty sharp and talented guy. You should of seen him ripping Hookipa on a wave windsurf board back in the day...he is talented, minka...