Splitters keep air from getting under the car. Diffusers accelerate it out behind the wheels. But without side skirts, air can get sucked in from the sides of the car and decreases the effectiveness of these parts.

Side skirts ensure the low pressure area extends much further under the car, and that means less lift. But these CFD-optimized parts are a lot more complex than the simple horizontal board you usually see.

This is the "full kit", which means these skirts include a carbon fiber vertical fin to control the turbulent air coming off the front tire. There are also two under-skirt aerofoils - one to deflect air from the front wheel outwards, and a second to deflect air away from the rear wheel. There's also a spat in front of the rear wheel to cut down on drag. 

The effect is a considerable extension of the low pressure zone under the car, which means less lift and more traction. The CFD rendering shows the difference between a Miata with the RGR splitter, spats and diffuser - the dark blue is the area of lowest pressure, which is what you're looking for.

Made of milled Delrin, carbon fiber and aluminum. Installation is fairly easy.