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 it is possible to access the Sacred Grove early. It is then possible to continue through the Grove and obtain the Master Sword long before it was meant to be obtained, and with it the abilities to transform between human and wolf, warp outside of Twilights, enter Lakebed Temple without the Zora Armor, and defeat enemies more quickly by dealing more damage than the Ordon Sword can.
Proceed normally through Faron Twilight to the N. Faron portal fight, collecting any tears routed in. (Routes that complete Faron Twilight and perform EMS leave some tears behind to be collected after obtaining the Master Sword.)
Stand near the tree root and lure the lone messenger on the left (GCN) as close to the wall as possible. Begin a Midna charge and release it while the messenger swings at Link. For this to work, Link must be in the correct position underneath the messenger, the attack's angle towards the wall must be correct, and the attack must be released at the correct time in the messenger's swing. Also, the messenger has two distinct types of swing animation, and while the trick can be done with both, it is much easier and more consistent to do so with one than the other. See video.
Once Link is on the wall, follow the wall around towards the Sacred Grove. When Link is close enough to the Sacred Grove entrance, it will appear on the minimap. Dash off the ledge to jump down into the area right in front of the Sacred Grove entrance. If this jump is not properly angled or is at low speed (no dash), Link may void before landing.
Obtaining the Master Sword early does lead to a number of dead ends. However, solutions have been to discovered for all of them. These include:
Fixed EMS is obsolete, but it is still possible and kept here for historical reasons.
While not used in any current route, it is possible to complete Goron Mines prior to obtaining the Master Sword while still obtaining the Master Sword early. 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. Fixed EMS was used until it was discovered that the spinner can be used to bypass this wall and 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. The game can now be continued normally, properly unlocking and beating Goron Mines and getting Barnes' Bomb Bag.
Later, go back to N. Faron and do the bomb boost to Lost Woods to re-enter Sacred Grove and obtain the Master Sword.