Q: What does a rear diffuser extension actually change on the Model Y Performance?

It lengthens the working surface of the factory diffuser and adds two central strakes that keep underbody airflow organized. Extending the exit lets the air expand more smoothly into the wake behind the car, which improves pressure recovery, adds rear downforce where pressure is lower, and delays separation to trim drag. In plain terms, the rear end feels more planted through fast sweepers and highway lane changes.