The TdrDelay is the Timeout Detection and Recovery (TDR) delay. Timeout Detection and Recovery are incorporated into Windows Vista and Windows Server 2008 and can detect when the graphics hardware or device driver take longer than expected to complete an operation. When this happens, Windows attempts to preempt the operation, and restore the display system to a usable state by resetting the graphics adapter. This may be set to 0 to disable the feature all together (I haven't tested this, use at your own risk). Other OSes may have TdrLevel as well (Windows 7 does not) or other TDR specific "features" you may tweak.