Get Item Manipulation (also known as GIM) is a memory manipulation technique which can be used to modify which item you receive from an item drop on the ground by modifying the Get-Item value. Every method of getting GIM in this game work the same way as int OoT. The most common method involving getting Walking While Talking on an item drop, which gives text when you obtain it and also the ability to move while the item is in your face.
After getting in the GIM state, you just need to go near a chest and touch it (so that your A button reads "Open", in whatever language) 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 (as said before, a Heart Piece for example) 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. Below are the several methods of getting GIM:
This is the easiest method to get GIM in the game, as you simply need to get out of a crawlspace and while the camera is fixed pick up an item drop. This will make the camera mess up, making Link unable to walk while enemies around can; get hit by an enemy to be able to move with the Get-Item on you, hump a chest to change the value, then jump into the water.
This method is very similar to the crawlspace method since it's based on a weird camera state. If you drown or just die in water the camera will mess up and you'll have to pick an item drop quickly and get touched by something afterward. For some reason, if you die out of bounds, the camera will get weird for a very long time, making GIM easier to get.
This second concept requires the iron boots. When you die in the water with iron boots on, there will not be an animation for Link standing up. Taking advantage of the weird camera state, simply pick up an item drop and quickly after picking it up go into the Items Menu and remove the iron boots from your X,Y,1,2 buttons. This will make Link's surfacing animation interrupt the Get-Item animation.
A very old method to start GIM is to use the swim on land glitch. To activate the glitch, you need to be in the water, get near the shore of the water just before surfacing, L Target looking to the sides and sidehop outside the water, pay attention to the items icons (they should be usable while Link is still in his swimming animation) if this is the case quickly open the items menu and remove the iron boots from your X,Y,1,2 buttons. Link should be swimming on land now.
Now if you dive and pull out the hookshot/bow, you'll stay in a diving state. The A button will display the numbers from 2-8, in most cases it will just display 8. You will have a ground jump stored, simply get out of it by a backflip and run towards an item drop to activate GIM.
When the game is under the state of GIM, some oddities occur like bombs and bombchus freezing midair after you pull them out.
Link can only open doors with knobs, sliding doors won't open.