The Scene Portal is a variation of the Swinging Door, i.e. a dual-scene mechanic where a player in the middle stands or otherwise ends up between two players, then alternates between both of those outside players in playing different scenes.
The one twist with a scene portal is that any of the middle person’s lines can be relevant to *both* scenes. If the middle person says a line to one player, the outside player can respond to that line as if it were said to him/her, and the line becomes relevant to their scene. This of course can get quite fun the more out of context the line is to the outside scene.
This doesn’t mean that *every* line the middle person says in one scene is relevant to the other player’s scene. A line only becomes relevant to the other scene if the player outside of the scene responds to it and swings the middle person around.
In effect, the Scene Portal is a swinging door scene where any given line can provide a portal for the central character between the two scenes. The portal definitely spans space, and can also span time.