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It's very complicated. Deflecting air downward at a tab will create an upward force on the attachment there, but if the flow then impinges on the tonneau, there will be a local downward force at that point
The net result (in a simplified world) would be a tendency to change attitude or squat. There may not be much "lift" ie. a sum upward force on the truck. Actual lift overall is different than a rotational trim change force.
Deflecting air upwards with a tab would create the complementary situation. The tab attachment has a downward force. Below and behind it is decreased pressure tending to pull upward on the tonneau and the pair would tend to rotate the truck toward a negative attitude.
HOWEVER, it seems like there's so much else going on with eddies and turbulence in the pickup box and vortices off the back, I feel like the above is just one trivial and incomplete mental picture of my own. I think it does happen, but how much, when and where.... nope. Just have to try things and see.
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