By starting a file in BiT as the thunderhead opening cutscene begins to play, you can open the thunderhead on that file before you are supposed to. This trick skips everything between learning Ballad of the Goddess to the Faron Silent Realm, including fighting the Imprisoned and repairing Scrapper.
In order for a file to trigger the thunderhead cutscene in BiT, it must have two conditions satisfied:
Once you have such a file, selecting it while in BiT will cause the thunderhead cutscene to play as soon as it can. If another dialog or cutscene is playing, the thunderhead cutscene will not play until that one is finished. Things that can delay the thunderhead cutscene include:
As soon as you select your file, the thunderhead cutscene is queued, and as soon as the first glitched cutscene is done, it will play. So by loading your file right before the thunderhead cutscene starts, you'll trigger the Early Thunderhead glitch. The window for this trick is about 1 second, as that's how long it takes the screen to completely fade to black after you start your file.
With this method, you will be loaded back where you saved, either in the Deep Woods or at the save prompt you used.
This method queues the thunderhead cutscene behind the saving dialog instead, and allows you to teleport back to Skyloft by doing a BiTSave at the same time. That makes this method faster for Any% speedruns; it is usually activated from the save prompt after finishing Skyview Temple.
If your timing is off while doing this method, the BiTSave will still happen, making the trick difficult to reset. You can try again by saving at a statue in Deep Woods, or waiting for a later save prompt.
This method is a little different in that you do not have to be saved in Deep Woods or at a Save Prompt; this version usually saves outside the Earth Temple. This method is used in 100% speedruns, where it saves only a few seconds by skipping climbing the Light Tower and and playing the Ballad of the Goddess.
This method queues the thunderhead cutscene behind the shed-opening cutscene, and will respawn you back in Eldin, since you did not do a BiTSave. You can then return to the sky and proceed directly to the thunderhead to open the Isle of Songs.