Ritual phrases. That's how you resolve this sort of thing. Ritual phrases come from some RPG--I'm not sure which, perhaps it's just a PbtA thing--where invoking certain words specifies a procedure that is to follow.
@Mannahnin and
@Burnside have achieved the same effect.
For my own game, the ritual phrase is, "
Previously, in the Desert..." When I say that, the Session Proper has begun. We don't have a ritual phrase for ending session (though I might see about developing one), since that's usually a matter of "we have to break within 30 minutes of the usual ending time for people who have RL things to do." Sometimes, the ritual phrase changes because the party
isn't in the titular Desert anymore; as an example, while visiting the perpendicular plane of Zerzura (my adaptation of the very excellent
Gardens of Ynn), I have said, "
Previously, in Zerzura..." I didn't make any changes during their (very brief) visit to Thelitaf, an island off the coast of the Tarrakhuna, but if I were to, it would likely be "
Previously, in the Sapphire Sea..." or "
Previously, among the Ten Thousand Isles..."
Invoking ritual phrases in this way establishes a mood. It sets a clear demarcation of an
event happening, and by
being a ritual--something memorized and practiced--it induces players to genuinely
think differently after it's been said. Like how practicing a piece of music enough times can lead to being able to replicate the motions even without seeing the notes before you.