[aerohead]

I think it was Sport Truck Magazine that interviewed the industry aero gurus around early 90s.
The operation of the 1/2-tonneau was explained as :
*the low pressure of the locked-vortex is telegraphed under the cover to the inner face of the tailgate.
*The delta-P across the gate is more favorable than with a closed,100% tonneau.
*It's this delta-P which gives the aero advantage.
*it's half the weight
*it allows access into the bed