Although the disk idle time‑out alone can be configured with the "Turn off hard disk after"
setting to favor power savings or performance, some Windows systems might exhibit very small amounts (bursts) of disk activity separated by relatively long amounts of disk idle time. This pattern of disk activity impacts system power savings because the disk is powered up periodically. The disk then remains in the spin-up state for at least the disk idle time‑out, even if the amount of disk activity that caused the disk to spin up is very small.
The disk burst activity time
addresses these scenarios by allowing the administrator to configure an amount of disk activity that will be ignored as the power manager accrues disk idle time for the "Turn off hard disk after"
setting. For more detailed information
about disk idle detection and disk burst ignore time-out purposes and settings, you could download the Windows Disk Idle Detection
Word .doc file from Microsoft.