Yes, a seat harness is a bit more effective for larger sails and slalom sailing.
you can still rotate your hips with either type of harness, so that part works.
I was out yesterday on the Isonic 145 and Apollo and paying very close attention to what I was doing to change the board from across the wind to upwind. I'm going to try to do the same thing this afternoon and see if there are any things I need to add.
I've been doing it for so long, that I need to recognise exactly what it is that I do. 
Yesterdays analysis seemed to suggest that "seetihg the hips" is less important than liftign with the front foot (which controls the roll angle).
But, I don't think you can effectively roll the board with the sail fully raked back and sheeted in without "setting your hips" at a different angle than your upper body.
What it feels like is that you are sailing along on a beam rach with good speed and then without changing your upper body position, or the rake/sheeting angle of the rig, you pull up and out with the front foot and increase the pressure across the top of the fin with your rear foot.
Upper body and rig stay the same, but the board rools slightly to leeward (due to lifting and pulling upwind with the front foot) and you push quite hard across the top of the fin to get it to max. horizontal lift and keep it there.
So, the description still fits as what you are doing is basically twisting the board (relaitve to the rig and your upper body) so the nose kinda pulls upwind and you push the tail away with the back foot over the top of the fin. 
Board changes course to much higher upwind without losing much speed and you are then sailing upwind completely on the lift from the fin.