Apparently no consensus whatsoever about this he hee - a bit funny actually
Some says it is for having more pressure on your front foot to keep the wing down.
Some says it is for having more pressure on your rear foot to keep the wing up (as you ride with lower speed upwind).
Some says it is for having your front foot straight to convert all gusts to upwind/speed.
Some says it is for having your rear foot straight, and flex on the front foot.
Some says it is for having both feet straight and locked, and twisting for upwind drive.
The only thing I think we all agree on, is that twisting your body (hip and or upper body) "upwind" drives as high upwind as possible.
Personally, I think the latter is the reason, namely like Dave says, locking both feet will make good riders able to convert every small and big gusts or windchanges to more speed or upwind angle, and twisting drives everything upwind