I gave that my best shot at
Permalink #24 with the half-bell. Think about a NACA duct. Vortexes generated by the curvature of the duct sides pulls air down into the throat of the duct. This acts like a reverse NACA duct, pulling underbody air in toward the center and up into the bell.
For balance, here's a different approach, from Bonneville:
This is also a bell — the roof has some curve to it. But it progresses from the full width, narrow inlet to an outlet that is ~4-5x the opening. I think this slows the air down and decreases it's static pressure to match the wake. But... I'd want to get aerohead's opinion on whether that's even wrong.
and again:
2nd from the top left is the High Momentum Mudflap. It's boat tail shaped.