It's an old arguement that has been going on for years now. Seat or waist, what's it all about right? In the end, it's just a hook.
Thats true. I mean, the only thing it does is tranfer the pull of the sail from your arms to your body allow a more active steering during the ride, a closer trim and thus more speed and agility. But why 2 kinds?
Looking at it from a biomechanical point of view you could defend the thesis that waist harnesses are for those who need to move a lot, like wave and freestylers and seatharnesses are for those that need to transfer as much weight from their body to the sail thus allowing for bigger sails in higher winds.
A seat harness covers the lower part of the upper body where the only mechanical movements that need to be made are turns of the torso in a freestyle move which is done unhooked, and bending backwards and forwards allowing for respectively acceleration due to weight transfer and controle of the sail in overpowered conditions. Mind you this latter aspect is based purely upon the riders expertise and the pull of his sail, as you'll see this latter pease of info is what really differenciates seat from waist.
Why is it harder to acchieve this with a seat harness then? Looking at the design seat harnesses are basicly an external prothesis of your rear with the one exception that due to it's rigidety the torsion movements is somewhat limited. The attachment of your 'bum muscles' to your leg muscles limits freedom but maximizes the potential of the rider to transfer weigth because of it's low point of attachment to the sail. Bending your legs becomes a bit harder to do in a seat harness reducing (in a trained individual with propperly tightend legstraps) the reflex to 'sit down' (this one if @ Joe, maybe you're feeling isn't such a good thing for your surfing). In order then to accelerate a rider has to find a more effective way to quickly transfer weight to the sail and has to do this by moving his gravitational point backwards (leaning) creating a more effective transfer of weight to the sail and also reducing the pressure on the boards thus reducing friction of the board vs water.
This is the main reason why seat harnesses are said to be race and freeride harnesses wheras waist harnesses are said to be freestyle/wave harnesses. Mind you these assumptions are not definite and you'll even see variation amidst pro riders. Peter Volwater for example is convinced that he can do everything with a waist harness and does so quite succesfully. On the counterpart there are some freestyle and waveriders that prefer the low harnessposition and use a seatharness.
Seat or waist is no absolete story but from my personal experience it does pay to try both in both diciplines. I personally don't like the seat harnesses because it kills my groin after a good overpowered slalomsession but i do feel faster in one and i feel like i have more controle. On the other hand, when i freestyle i hate the seat and i use my waist and do so for wave as well. I have to admitt though, that 9/10 times that i sail, no matter what, i'll use my waist harness because that is the type of harness i learned to ride with so there you see that conviction is also partly conditioning.
I hope this settles it
Haiko, AKA crazychemical