It's not a throttling technique, and without diving too far into the technical details of what asynchronous computer operations are, essentially what it allows is for any websites that use the Flash player plugin to utilize the asynchronous rendering capabilities of the Windows OS.
Specifically this allows multiple instances of a video plugin to work in tandem without causing crashes due to any synchronicity issues. As an example, if 2 tabs are open with YouTube playing a Flash based video and they both tried to write their image data to the screen at the same time (regardless of visibility to the user), both tabs could crash.
The async nature allows buffering to occur (much like a print queue for a printer), in addition, if a specific drawing surface is not visible to the user (like a background tab playing a YouTube video), the plugin simply won't "draw" anything until it gets the notification that its drawing surface is actually visible to the user (i.e. active drawing surface); this non-drawing action doesn't consume any additional resources like a video actually drawing to the screen and thus you can see a slight improvement to performance.
Hope that can help.
