The Spiny Shell gained wings, allowing the item to fly over the course while avoiding all the obstacles around, as opposed to its former appearance, when it would be destroyed when it hits an obstacle. After firing one, it would fly over the track, homing in on the current contestant in first place. Once it reaches its destination, the shell hovers briefly over its target and then lands on the kart, blowing up everything in an area around the kart in an explosion.
How to Cheat It
In order to avoid the blue shell, you nedd at least 1 mushroom. Timing is the key factor, as when the blue shell stops hovering, use the mushroom as the the explosion will be behind you.