Early Master Sword (EMS) is a sequence break that uses a super jump off of one of the N. Faron Twilit Messengers to reach an out of bounds ledge from which you can access the Sacred Grove early. You can then continue 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 after beating Goron Mines, the bridge CS will play once you approach the bridge, making the portal obtainable and solving this problem.
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 that 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, return to the skull kid fight area without collecting the Master Sword. Then savewarp to return to the start of Sacred Grove. Leave the Sacred Grove and void out until reaching a Game Over, then select "Continue" to respawn 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.
Later, you can go back to N. Faron and do the Bomb Boost to Lost Woods to re-enter Sacred Grove and obtain the Master Sword.