Activate a standby database and then revert to standby

[Massimo Ortensi <mortensi@xxxxxxxxxxxxxxx>]

I need to activate (open in read and write) a standby database, perform 
some action and then revert it to the previous status of standby and 
thus starting again loading wals from when it stopped.

How should I proceed to be sure it's possible to get "back in time" to 
the standby db ?
It's a huge DB and can't afford restarting sync from scratch