A Mirror Jump involves hopping a ledge and using the Magic Mirror at the same time. The behavior of Mirror Jumps varies depending on if the player is on the overworld or in the underworld.
Overworld Mirror Jumps
To perform Overworld Mirror Jumps, the player must be in the Dark World and use the Mirror at the same time a ledgehop is activated. From the instant a direction is held against a ledge, the game will start a 20 frame countdown where Link is stuck in place moving against the ledge. On the 20th frame he will begin his jump. The Mirror must be used on this 20th frame for a successful Mirror Jump. This will warp Link into Light World with Link's position shifted by 1 pixel. If the player Mirrors before the 20th frame in the ledgehop countdown, Link will be put in the Light World but without having his position shifted any. If the player Mirrors after the 20th frame, then the Mirror won't work and Link will just jump the ledge in the Dark World.
Doing this one or more times can put Link Out of Bounds, allowing him to move to unintended areas.
Underworld Mirror Jumps
To perform an Underworld Mirror Jump, players must be indoors with a dungeon loaded. If the player is indoors with a cave loaded, the Mirror has no function when used.
Just like with Overworld Mirror Jumps, the player must use the Mirror at the same frame a ledgehop begins. However, unlike on the Overworld, this can be combined with a quickhop (assuming the player has the Pegasus Boots) to make it easier. Simply presson the same frame while walking against a ledge. Without boots, the same technique used for Overworld Mirror Jumps must be used.
After a successful Mirror Jump, the player will be stuck in a sliding animation moving towards the direction of the ledge that was used. Ordinarily, left and right ledges are bad for this, as Link will mirror back to the dungeon's entrance and be stuck moving against the doorway's walls indefinitely (this is a softlock, with the only meaningful action available being to Save and Quit). However, these ledges can work if the dungeon entrance wasn't a doorway (eg. Skull Woods pits). If a Mirror Jump keeps Link in the dungeon, he will continue sliding until he hits an enemy (which can activate EG immediately). If no damage is taken, a softlock in the sliding state is likely, where the only action the player can do is Save and Quit. If the player does a mirror jump on a south ledge, then Link will exit the dungeon and slide south some on the overworld. If the Moon Pearl has been collected, the sliding animation will be cut off and the player regains control of Link and EG will be armed.