The phrase "flat underbody" refers to a flat or level surface underneath a vehicle. It means that the bottom part of the vehicle, such as the chassis or body, is designed without any bumps, curves, or protrusions. Full definition
A completely flat underbody, extra large rear diffuser and rear airbrake add to the heightened dose of performance, although make sure you and your passenger have a brace of helmets if you intend to fully exploit the supercharged engine's power. (evo.co.uk)
The front splitter and side skirts help reduce drag, while the rear wing, rear diffuser, and a nearly flat underbody create meaningful downforce at speed. (automobilemag.com)
The car's body is made of lightweight materials, and flat underbody panels and a grille shutter reduce aerodynamic drag. (automobilemag.com)