This technique allows you to pull out a Deku Stick away from a flame and still have it become lit. This effect can be carried anywhere Link can normally go with a Deku Stick.
This is achieved by detaching the sword's hitbox on the burning part of a torch. Technically this does not "store" a flame at all, it just looks that way. When Link draws a Deku Stick, the game immediately checks to see if the hitbox of the stick is colliding with a flame graphic, and if it is then the stick lights. This holds true even if the hitbox is separated from the Deku Stick.
This also means that if Link moves far enough away that the flame is no longer loaded, the Deku Stick will not catch on fire when pulled out. However, it will work again if you simply come back close enough to reload the flame. Swinging your sword or leaving the scene will reset the hitbox, cancelling flame storage.
There are two known methods to detach a sword's hitbox on a torch:
If it worked, you should be able to move away from the flame and the next Deku Stick Link uses will catch fire.