A little context behind my answer.

Among my friends, we have this weird but I guess clever safe word tradition. So you know how, when someone is dreaming, if they question the dream (e.g. saying “this is a dream” or “this must be a dream”), they wake up? We decided to use that as a “quitting word” for everything. If we’re making a YouTube video and we say “this is a dream”, for example, it’s the secret cue to cut the movie. If we’re playing some kind of role and we say the phrase, it means come back to base reality. So on and so forth. And while none of us are very sexually-minded, sex we decided would be no different. If we were to have sex, a phrase like “this must be a dream” would mean to stop the session.

So then it was asked one day, how do we do the opposite? How do we cue a session of sex, video-making, etc. to start? We say the opposite. We signal immersion with something like “where are we, is this real”. That is our unsafe phrase. Different nonverbal circumstances/signals would cue what exactly we refer to.