essentially the up position gives you controle, the down position gives you speed if you have the technical abillety to controle your board/sail under such circumstances. This is basicly why wave sailors use waist harnesses and speed sailors use seat harnesses.
personally i'm a fan of waist harnesses because of the controle thing. Easily hooked in, easily hooked out and if you're overpowered enough you still get a decent speed. And in the end, there's no speed without controle so i basicly never use my seat harness for smaller sails or overpowered conditions (ie i use it once a year, i have no idea why i still have it ....).
ps nice nick
i used to be called crashtester untill i found guys that wreck 4 times as much stuff as me