Reverse BiTMagic (RBM) is a glitch that allows scene flags from areas in Back in Time to be applied to the Area for a saved file. This allows for several sequence breaks and skips.
A scene flag is any sort of permanent or persistent change that can occur to an Area's environment. Examples include pushing logs off of cliffs, blowing up piles of boulders, or opening gates. Every Area in the game has a segment of memory that stores the current values of its scene flags. There is also a segment of memory that stores "current" scene flag values; that is, the values of the scene flags for whatever Area the player is currently in.
What Reverse BiTMagic does is Start a file during BiT, which loads the Area scene flags for that file into the current scene flag memory. But any scene flag which is set in BiT during the fadeout is also applied to the current scene flag memory. Thus the scene flag from BiT can be applied to the Area where the started file was saved. Making a change in that area then updates the Area scene flag memory for that Area, allowing us to permanently modify the environment of any Area we can save in.
To perform an RBM trick:
The exact details for where you go during BiT and what action you take as the fadeout occurs will vary from one RBM trick to the next, and some are easier than others. For details on a specific RBM trick, see the trick description on its relevant page, linked below.
RBM is a very flexible trick in the sense that any scene flag we can set anywhere in BiT (including Faron BiT) can be applied to any Area that we can save in. The uses of RBM, therefore, are only limited by what flags we can access in BiT, and what those flags correspond to in each area.
There are several other uses of RBM as well; the listed ones either were or are used in some category's speedrun route.
Using RBM, it is possible to get the scattered Gratitude Crystals on Skyloft to appear during nighttime BiT via BiTMagic. This enables a few useful RBM tricks, since nighttime BiT can only be accessed with a file saved before obtaining the Hero's Tunic, which is normally too early to obtain the ability to collect Gratitude Crystals.
For this RBM, you need two things:
With the appropriate setup, activate BiT. Enter Batreaux's house during BiT with the target file selected, then start the target file at the same time as clearing Batreaux's final textbox (which is the sixth textbox after the text option). Once your file loads, resave it on Skyloft, then continue with your run as usual.
There are several scene flags tied to nighttime crystals on Skyloft. Useful RBM tricks that employ these crystals are:
A community effort has documented many scene flags in the various Areas of Skyward Sword. These flags have been documented in the spreadsheet below. If you want to know if something can be RBM'd, look for that event in the correct Area tab. If you find it, look up the corresponding flag event in Skyloft (or Sealed Grounds/Faron Woods/Lake Floria for Faron Back in Time).