Early Master Sword (EMS) is a sequence break that uses the N. Faron Twilit Messengers, coupled with a super jump, to reach an out of bounds ledge from which you can access the Sacred Grove early. The player can then continue on and obtain the Master Sword long before it was meant to be obtained, and with it, the ability to transform between human and wolf.
Early Master Sword used to lead to a dead end involving the Eldin Bridge portal being unobtainable, thus preventing the bridge piece blocking access to the Cave of Ordeals from being warped away (rendering the three poes inside unobtainable).
However, it has been found that if you blow up the Lanayru rocks before beating Lakebed, and then return later on after beating Goron Mines, the bridge CS will play once you approach the bridge, making the portal obtainable and hence being able to warp the bridge and enter Cave of Ordeals.
Note: Despite being almost entirely obsolete, Fixed EMS is still possible and still kept here for historical reasons.
While not used in any current route, it should be noted that it is possible to complete Goron Mines prior to obtaining the Master Sword while still obtaining the Master Sword early, thus overcoming the major obstacle of entering Goron Mines. Normally, once the Master Sword is obtained, the sumo event required to unlock Goron Mines breaks due to the area being in the wrong state. This makes it impossible to remove the invisible wall blocking Goron Mines normally. It was discovered the Spinner can be used to bypass this wall; furthermore, it was later found that the sumo event's text associated with beating the event can be triggered via Text Displacement.
To do this, execute EMS as shown above, but after solving the Master Sword puzzle and opening the door, instead of collecting the master sword go back to the skull kid fight area. Once here a savewarp will bring you back to the start of Sacred Grove. If, then, you leave and void out until Game Over, selecting continue will re-spawn Link all the way back at the start of Faron. You can now continue the game normally and properly unlock and beat Goron Mines, thus allowing you to get Barnes' bomb bag if needed.
Now you can go back to N. Faron and do the Bomb Boost To Lost Woods, once in lost woods due to already completing the Skull Kid chase earlier and solving the puzzle as wolf earlier means you can now collect the Master Sword Early, while having already beaten Goron Mines and obtained Barnes' bomb bag.