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gregjet said..
If you put the straps around the bar with a simple "under-return" the load on the buckle is the full belt tension load. If you wrap the strap completely around the bar (at least once, but 5 times will give you full capstan effect) , the load tension on the buckle reduces towards HALF, and with wraps at both ends tends toward a quarter. You are less likely to get loosening amd you straps will also last longer ( less surface tearing).
BTW the lift on a board is not just the windscreen diversion load but also the fact that a board is almost a perfect lift generating wing when nose down. Take a good look. Unfortunately turning it nose up on bars doesn't give you much downforce, because the bars underneath disturb the flow that would help.
I don't wrap the strap around the rack multiple times because one side won't tighten properly. The side with the buckle ends up taking all the load as the other side won't fully tighten due to the wraps so the load is not spread over both sides. It also takes a lot longer, I prefer using the "oh S**t" strap if I want to have less tension on the straps and roof rack.
When you say nose up creates downforce, I assume mean tail forward with nose up in the back, right? That way it creates downforce in the back like a spoiler but not in the front where uplift can be a problem at high speeds.