This is an Edit technique: while a scene is played, another player steps forward and tells the audience what's next. This takes some training (and accepting) as the players in the scene should instantly drop whatever they were doing/saying to make room for the voice-over.
The voice-over may indicate where the next scene happens, as in "meanwhile, in the park...", or she may offer additional information about what was going on in the scene, as in "What Charles did not know, was that the necklace he had bought for his wife, had a long and horrifying history...".
Either way, once the voice-over is over, either the scene that was underway continues, or a new scene starts (or any previously interrupted scene continues). In either case the information provided by the voice-over is incorporated in the storyline.
This edit technique reminds us of the Typewriter game handle.
Mistakes are the portals of discovery --- James Joyce