Get Item Manipulation (also known as GIM) is a memory corruption glitch which can be used to modify which item you receive from an item drop on the ground by modifying the Get-Item value. The current method we use to get GIM in this game, is by diving as human link to get an item drop and then interrupting it with a cutscene. After getting in the GIM state, you just need to go near a chest and touch it (some also say hump) and the Get-Item value changes. The item drop doesn't change the Get-Item value, it only depends of the chest you hump (note that it's not only chests, you can also hump Heart Pieces, as long as you don't grab them which could make you lose the GIM state.) If you're not humping a chest (or a heart piece) the Get-Item value will be set directly to the item you touched and so in that case, you'll get a heart piece, that's a way to duplicate items.
So far, these are the results of chests we have been able to do GIM with:
Dungeon Map/Compass Chest: Remove Ocarina (sending Link back to the 1st cycle of the game.)
Boss Key Chest: Obtain the Ocarina back.
Key Chest: No Apparent results (need to check other places to see if anything's changed)
50 Ruppie Chest: Replace the Ocarina with another item (in this game we cannot see or use any item placed in the Ocarina slot, so we're not sure what item this could be.)
20 Ruppie Chest: Obtain one fishing pass.