 
        // ntifs.h
VOID PsRevertToSelf();
View the official Windows Driver Kit DDI referenceNo description available.
The PsRevertToSelf routine ends the calling thread's impersonation of a client.
A server thread can impersonate a client by calling the PsImpersonateClient routine. When the thread is done impersonating the client, it can call the PsRevertToSelf routine to end all impersonations.