Short answer: Makes traffic dissapear.
Long answer: When you do that camera flick the game marks the cars that have just spawned in front of you as "out of your view" thus despawns them. This means that you can't despawn all cars by doing this, only the ones that have just loaded or are in the process of loading. This could also backfire and end up spawning traffic right in front of you so it's not a magical strat but it works in your favor most of the time.
The GTA:SA community has been deteriorating over a long period of time. Many top runners are not currently present on the leaderboard. This issue has not been seriously addressed yet, but an attempt will be made to remedy it now.
As of this news post, the San Andreas leaderboard will function in
