I believe the idea of the Power Now is not so much to direct air flow, as to reduce the turbulence that occurs when the column of intake air contacts the exposed face of the partially open slide.
As the “vane” resides very close to the slide face, turbulent air above it (at part throttle openings) is restricted from mixing with the relatively laminar air flow below the vane.
In theory, the flow is improved at throttle openings where the slide cutaway is below the vane (potentially the best available flow is when the cutaway is parallel or in line with the vane) and this is why they recommend a slightly leaner mixture.
The less turbulent, more laminar, higher velocity air flow creates a stronger than usual vacuum signal (negative pressure), pulling more fuel thru a given jet size.
Or… I am completely full of shit, and made all this up as I went.