Hi Ron,
(second attempt at posting this...
)
Here's my take on the "concept",
If you look at the things that cause air turbulance, etc., the plate edge as-is would actually create a drag point or bubble on the edge forcing the air around the edge and more into the groove.
By taking that edge away, you are correct, it allows for more air to pass freely through the TB. The downside of that is that more air gets past the Groove.
On the other hand, by shaping the underside of the plate, it frees up the "groove modified" air to exit the groove with less interference from the plates trailing edge, such that it is.
As I said, my take on it anyway...
Karl Fortner
Tacoma, Wa.