At first, the player who pulls this item will gain a temporary speed boost and off-road similar to that of a Mushroom, but after ten seconds, it will shock the player in a way similar to that of getting struck by Lightning.
Passing the Thunder Cloud
It is possible to pass this item before being shrunk, upon passing, the item will gain a small duration increase and will be passed to the player after a direct hit. Because of game ping (or lag), it's possible to pass them in very bizarre ways sometimes.
Why Is The Item Only in Mario Kart Wii?
The Thunder Cloud is one of the most hated items in the game by the community, all it did was punish you for being in the middle of the pack and give you an obstacle that wasn't appreciated in the majority of circumstance, hence this item is very unlikely to return in the future despite Nintendo never commenting on it