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 and not being able to warp the bridge that blocked Cave of Ordeals away to enter, which meant the three poes inside couldn't be collected.
However it has be found that blow up the lanayru rocks before lakebed and then return later on after beating Goron Mines (By entering with the spinner later on) 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.
Although no longer used in any categories it should be noted it is possible still get the Master Sword before intended while beating Goron Mines first, normally with EMS the only method of entering Goron Mines is to use the spinner to clip through the goron guards.
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, and then if 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 beat Goron Mines, this allows 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 being able to have Barnes' Bomb Bag, whereas with normal EMS this wouldn't be possible to collect until after Arbiter's Grounds for access to Goron Mines and hence Barnes' Bomb Bag.