The Spiny Shell is a racer's worst fear, as it always flies towards the leader. When it reaches it's target, it hovers for a moment, then smashes into the ground, causing an explosion that affects all racers within range. It is a blue shell with spikes on the back of it and has wings in some games. They are also known as blue shells.
Using the Shell: There isn't much to know when to use the shell, usually you are about sixth or below when you acquire this item, and this usually doesn't do much to help your own lead, unless the racers from first to fifth are caught in the explosion. It avoids the other racers by flying above them. You may decide to hold onto it until you're in second or third, but generally you can get better items to help catch up (such as a Mushroom.)
Avoiding the Shell: There is only one way to block it: You must have a Mushroom, and when the shell is about to hurtle toward the ground, boost, because it will not be able to follow you once it drops from the sky. This is a very hard trick to master and requires excellent timing, as well as a bit of luck (Having a Mushroom while in first isn't all that common).
The Spiny Shell has appeared in Mario Kart 64 (1996), Mario Kart: Double Dash!! (2003), Mario Kart DS (2005), Mario Kart Wii (2008), and has also appeared Mario Kart 8 (2014). It wasn't until Mario Kart: Double Dash!! that this leader's nightmare had sprouted wings.
Blue Shells: Ruining Friendships Since 1996.