The hwndLockupFrame parameter contains the frame window handle of the lockup dialog. All HK_LOCKUP hooks registered with the system are called when the system locks itself up. All HK_LOCKUP hooks must be system hooks, not message queue hooks.
Application programs that create other lockup password input windows by hooking the HK_LOCKUP system hook can use WinUnlockSystem to force the system to unlock when another form of input is detected other than mouse or keyboard. For example, a pen gesture or some voice input or signature recognition.