[kach22i]

The underbelly angle must be kept very shallow to retain attachment (2 or 3 degrees if I recall correctly), otherwise turbulence results.

The diffuser fences/strakes allows a roughly doubling of inclination angle. The 10 degree angle I drew was considered undo-able.

Going off memory of sharing a coffee with a professional aerodynamicist about five years ago that had work experience with several automakers.

The fences work because air attaches to the vertical surface much the same way as the near horizontal surfaces, I suspect as parasitic drag hence people's words of caution about assuming a rear diffuser will always lower drag.