From the Super Mario Wiki
Lightning (also called Thunder Bolt, Lightning Bolt, or Shock) is an attack item found in the Mario Kart series. On its use, it summons Lightning from the sky, causing various effects.
Mario Kart series
In the Mario Kart series, Lightning is an extremely powerful item that racers usually receive when they are in (or near) the last position in a race. When used, Lightning will temporarily shrink all the other racers, making them slower and weaker, in addition to making them spin out and drop their items. It also temporarily makes the racers' voices high-pitched and sped up until being back to normal or getting squashed. It is also the emblem for the Lightning Cup.
In Super Mario Kart, Mario Kart 64 and Mario Kart: Super Circuit, if any shrunken players are driven over, they will be squashed and unable to move for a few seconds. While shrunk, the character's weight is equal to other shrunken players. If two shrunken players run into each other in Mario Kart 64, they will both crush each other.
In Mario Kart: Double Dash!!, Lightning can also scatter racers' items across the road when used. However, the item does not affect players that are currently indisposed (such as having just been hit by another item, or having fallen over a cliff). Also, unlike in Super Mario Kart, Mario Kart 64 and Mario Kart: Super Circuit, shrunken players will not get squished if they ram into normal sized racers. However, tiny karts will be easy to push around, even by the Bullet Blaster, the lightest kart in the game.
In Mario Kart DS, the effect of being hit by Lightning wears off much faster depending on the rank that the player is in. For example, someone hit by one while in 7th place will return to normal almost immediately, while those in 1st or 2nd place will remain shrunk for up to six seconds.
In Mario Kart Wii, as in Mario Kart DS, the effect of Lightning lasts more briefly the lower the rank the player is in. Also, players who get hit by other items can still be affected by this item. If the players are under the effects of a Mega Mushroom, they will shrink in size to a normal sized player. Unlike the other teams items in Balloon Battle, Lightning does not take away a balloon. This can also make players invulnerable to mostly, a Spiny Shell (Mario Kart). However this can prove to be risky if the Mega Mushroom power up is equipped as the player is open to attacks such as Green Shells, Red Shells, Bob-ombs or a Spiny Shell.
In Mario Kart 7, as in Mario Kart DS and Mario Kart Wii, the effect of being shocked by Lightning will be determined on the level rank the player is in, from last place decreased time and in first place will last the longest. As before, the theme of the current track will be altered and even hit by another item, the player will still go into effect. Any players that are shrunk will also be squished if run over by normal sized racers, an effect returning from Super Mario Kart, Mario Kart 64 and Mario Kart: Super Circuit. Unlike in Mario Kart Wii, when a driver is shot out of a cannon, the shrinking effect will still happen and the driver will fall while gliding midair.
In Mario Kart 8, the effect of this item is very similar to its effect in Mario Kart 7. In this game however, when a character is shrunken, it becomes more difficult for the player to perform tricks and to drift. Unlike in previous Mario Kart games, when a player is hit with Lightning, they will lose the item they are holding even if they are under the effects of a star. Like the first three games, when a player is shrunken by Lightning, and another one with the normal size drives on him, it will be squished.
Since Mario Kart 64 until Mario Kart: Double Dash!!, when Lightning is used, a sound like that of a tiny alarm is heard during its effect. In Super Mario Kart that sound is only heard when the Lightning is launched, and in Mario Kart DS, Mario Kart Wii, Mario Kart 7, and Mario Kart 8, the music in the race is affected when the player is hit by it. In Mario Kart 7, the extra beat that plays in 1st place goes out of sync with the music for a few seconds after the attack. Lightning's effect can be avoided if a racer has a Mega Mushroom, Star, Boo, or Bullet Bill item activated when the bolt strikes, or if they are flying through the air after being shot out of a cannon (except in Mario Kart 7 and Mario Kart 8).
Super Smash Bros. series
Lightning appears in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U as an item. It also appears as a Sticker. The item usually shrinks all of the player's opponents. However, at times it might backfire and shrink the player who grabbed the item. Lastly, there may be a case in which the item might actually make the opponents giant sized (it's just like the Super Mushroom, only it's twice as effective).
An interesting note to point out is that players that get shrunk by the Lightning will be smaller than when they touch the Poison Mushroom. Likewise, if players become giant from the Lightning, they will be much bigger than when they grab a Super Mushroom.
In Mario Hoops 3-on-3, Lightning plays much the same purpose, stunning all players and making the ball drop out of one's grasp. In Mario & Sonic at the Olympic Games it appears in the Dream Races where it also makes characters shrink and slow down. In Mario Power Tennis, Lightning appears as an item in the Item Battle in Exhibition Mode. It has the same effect as it does in the Mario Kart series. It shrinks the player's opponent(s), making them unable to run faster to get the ball. In Mario Superstar Baseball, it is one of the items resulting from landing the ball in the ? space. If the player receives that, they will switch with the leading player's coins with their coins. In Mario Super Sluggers, it appears as an error item, but only in Toy Field, where the losing player may receive it. If the player uses it, all fielders would get stunned by it while the player may get coins resulting from the stun. In Mario Party 9, Lightning appears in King Boo's Puzzle Attack. Lining three of them will cause a Lightning that will reduce the opponents points by one, and will give more damage to King Boo than other objects. It also appears in Mario Party Island Tour, known as Lightning Score Striker, it subtracts three from a opponent's roll and only appears in the Perilous Palace Path board.
Lightning cameos on the wrapper of Cashzap Candy in Mario Party 8. Lighting appears as an item in Mario Pinball Land as well. In Mario Party 9, Lightning appears in King Boo's Puzzle Attack. If the player lines up three or more bolts in one row, the player gets ten points. In Mario Party: Island Tour, Lighting is referred to as Lightning Score Strikers and subtract 3 from an opponent's roll. Lightning also makes a brief appearance in volume 6 of Super Mario-Kun. While struggling, Mario receives this item and then uses it to pulverize Donkey Kong Jr.. He then wins the race.
Super Smash Bros. Brawl
Super Smash Bros. for Nintendo 3DS / Wii U
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