The operation compensates for the local contributing scattering area. In the past, people tried to use the angle of incidence as a proxy for that area, but robust terrain flattening dispenses with that “angle” proxy and calculates the area directly from the DEM. So it compensates for what was formerly know as an “angle effect” but it does so by ignoring angles and integrating all the DEM area applicable within each (range, azimuth) sample coordinate.