I think it is all about tensioning the body / muscles so every pull of the kite/wind gets towards the board to be turned into speed, same as with the racboards ( kite and windsurfing), the more you turn your torso upwind the better your angle upwind is and kept because of tensioning all the mucles,
On the other hand, bending the knees etc, will result in less transportation of power towards the board.
Secondly you can hold more power as you can lean more against the board anbd pushing more power with less (power) loss towards it.
See if people are pulling ropes they also put themselves more on their side...to have more bodytension and better weight balance to hold the forces or pull...
Its only my opinion/experience...
Dude