I saw this in some player's YouTube video. They can drag a shell so that they don't get hit by other people's red shells. How do I do this?
You just hold down whichever button is normally pressed for shooting the item.
As soon as you release the button the item will then fire, and as far as I know you cannot stop holding the item behind you without firing.
This works with single shells, single bananas and possibly the fake power-up boxes.
It seems like when using the Wheel, it is by holding left or right on D-pad.
Tap Down once on the D-pad to hold it behind you. To shoot behind you, tap Down again. With triple shells, you effectively have a wall of death around you.