Down A is a glitch in the game that gives you the ability to walk through actors Link can interact with. You can also walk up seams you normally can't walk on, for example the slippery seams on the way to Goron Race.
Now you should be able to walk around and have "Down" shown in the A button. Jumping off a ledge, or grabbing a ledge of any kind will cause Link to revert back to normal. This can be worked around by backflipping or sidehopping off of ledges. Leaving an area will also revert Link back to normal.
When you perform Down A either being Swordless or having the B button dimmed (be inside a house) the buttons X,Y,1,2 will not be updated from their current state, meaning that if you had an usable item in one of these buttons, perform Down A and switch with an item you normally can't use, you're gonna be able to use it making this another method of Restricted Items
When you're in Down A mode, the camera is going to act weird and will always be above Link. Holding target will not place the camera behind Link but can mess up your movement while walking.
3DS users with the Circle Pad Pro or the C-Stick on the New 3DS can free the camera position while in Down A and can even make the camera clip through the ground below Link and through walls.
It is not required to remove the mask from Link's inventory while in Down A mode. As long as you target while holding Shield, you can still walk around. However, you will not be able to do anything either than walk, sidehop, and backflip. Rolling will cause Link to put on the mask you used to perform this.