[aerohead]

Yes, dead air pools within the perimeter, and the laminar flow above it just skims off it, as if it were a solid boundary. It must be airtight in order to work. No porosity.
The same happens under a car with the rear fascia, if it's properly configured.
Rocker panel extensions along with the rear valance can capture an 'upside-down' pool of air below the belly, leaving laminar airflow to skip over underneath it, as if a belly pan were above it. Subaru proved this with their 1985 Cd 0.29 XT.
I still have the CRX.