The primary purpose of a rear diffuser is to efficiently increase the downforce of a vehicle. As a vehicle is driven down the road high speed air is passing underneath the vehicle at low pressure.

Greddy X Rocket Bunny Version 1 Rear Diffuser Scion Frs 13 15
What does a rear diffuser do. But unlike humidifiers diffusers are meant to be used with water and essential oils. Operation rear diffuser the aft part of a car underbody is where a rear diffuser is usually located. The diffuser is usually located at the back of the car and its aerodynamic purpose is to increase the airspeed of the air flow underneath the car. Installed in an upper corner of the fresh food section the diffuser incorporates a damper to control the flow of cold air entering from the freezer. The main function of a car rear diffuser effectively increases the vehicles downforce and increases the cars grip on the road surface and reduce the aerodynamic drag. The air pressure beneath the car is affected by the diffuser so that the air pressure can normalize to ambient pressure in the diffuser as the car moves through the air.
Based on your question it seems like you might be confusing diffuser and spoiler. It works by accelerating the velocity of the airflow underneath the car. They do this in a number of different ways and the types of oils that people use will vary from person to person but the purpose is the same. A diffuser is like a small humidifier because it turns water into water vapor and emits it into the air. In some refrigerators the diffuser is manually. How does a rear diffuser work.
But as the rear of the car gets farther from the road the under car flow is more. The diffuser diffuses the essential oils into the the water vapor and the vapor carries the tiny oil particles into the air. The diffuser is on the underside of the car to the rear of the back wheels. 1 what does a diffuser do. When the rear of the car rides higher than the front rake angle the entire underbody can act as a diffuser. This helps increase grip and reduce aerodynamic drag.
The pressure under the car is affected by the diffuser so that it can expand back to ambient in the diffuser as the car moves through the air. Purpose of a diffuser diffusers are devices that are used to help release essential oils into the air. Its job is to smooth airflow and reduce drag. The spoiler on the trailing edge of the decklid helps smooth airflow and helps generate downforce.