This is a glitch that occurs through activating the flute menu with the use of Yuzuhara's Bottle Adventure in specific circumstances. It can be done on both the overworld and the underworld, with different effects depending on where.
Overworld Fake Flute
This is done by drinking a blue potion and activating an overworld screen transition on the same frame. This will call a normally functioning flute menu that will allow fast travel to the usual 8 spots on the overworld. However, unlike the normal flute, this can be used to access the flute menu in the Dark World. If used in the Dark World, Link will land in the Fake Light World, and the player will need to transition the screen to load the Dark World properly.
Underworld Fake Flute
Achieved with a green potion YBA through a inter-tile door or a blue potion YBA through an intra-tile door. This calls a flute menu which does not function properly. The player will be unable to fast travel around the overworld as the normal flute and overworld fake flute allow. Instead this is used for a couple of major glitches.