Windows® Internals, Sixth Edition, Part 2

Shutdown

If someone is logged on and a process initiates a shutdown by calling the Windows ExitWindowsEx function, a message is sent to that session’s Csrss instructing it to perform the shutdown. Csrss in turn impersonates the caller and sends an RPC message to Winlogon, telling it to perform a system shutdown. Winlogon then impersonates the currently logged-on user (who might or might not have the same security context as the user who initiated the system shutdown) and calls ExitWindowsEx with some special internal flags. Again this call causes a message to be sent to the Csrss process inside that session, requesting a system shutdown.

This time, Csrss sees that the request is from Winlogon and loops through all the processes in the logon session ...