If you have a Griffin PowerMate, this option will make it flash when new good messages arrive. The flashing will stop when you activate SpamSieve or your mail program.
To change the rate of the pulsing, open System Preferences and go to the PowerMate pane. Adjust the Rate slider in the Pulsing section of the window.
This feature is compatible with versions 1.6.4 and 2.1 of the PowerMate software.