This glitch is not speedrun legal, as it requires a premade file.
File duping is a way of copying over file information from one save file to another during BiT.
By saving during BiT without having a file selected in the file menu, the file with the highest possible number will be copied over the file that activated BiT. In other words, the game will copy file 3 over to the file that activated BiT. If file 3 doesn't exist, it will copy file 2, and if that doesn't exist, it will copy file 1. If no file exists, it will generate a BiT save file.
If you're in the file menu while duplicating a file, and the file you're overwriting is empty, the file will still say "New Adventure" or "Hero Mode" even though the file exists. Loading the file will play the intro cutscenes, and Link will appear at his bed afterwards, but Link will have all the items/quests from the file that was copied over, with the exception that the number of hearts will be set to six.
Using the file duplication glitch, it is possible to beat the game in under seven minutes. You need a Hero Mode file saved before (or after) the Demise fight at the end of the game, and this file should be in slot 3. You can start with a blank Hero Mode file in either slot 1 or 2. The route is as follows:
Note: This is not a legal speedrun, as it requires a file to be already made.
Load a New Adventure file and activate BiT without saving. Then perform a BiTSave. The file will still appear as a New Adventure file, but whenever you load the file, the intro cinematic videos play and then Link will be at his bed in Skyloft. If you save at any point normally on the file, the game will still say New Adventure, but your items and quests are still saved. Loading such a file will always cause the intro cinematics to play, and Link will always be at his bed in Skyloft, no matter where or how many times you save. All the progress you have made will still be saved, so you will keep all of your items.