The modifications centre around the car's sidepods, with the shape of the inlet and underbite corrected in order to improve flow to the radiators, coolers and ancillaries housed within. The reshaping will obviously have a bearing on the external flow stream too.
Red Bull has achieved this by widening the aperture and raising the leading edge of the underbite, which alters the aspect ratio, but shouldn't impact the volume dramatically.
This results in a win-win between its cooling demands and increasing space in the undercut beneath the sidepod to deliver more airflow to the rear of the car.
Source: Autosport