Thunderspiking is a technique that Pikachu and Pichu can use by launching an opponent upwards (usually from an up smash or an up tilt) and subsequently using Thunder to Star KO them when they are near the upper blast line, allowing them to score a vertical KO at rather low damage percentages. This technique is often used as a combo finisher (notably at the end of Isai's Greenhouse combo in Smash 64), or just for KOing. In Smash 64, the tornado hazard on Hyrule Castle can be used by Pikachu players to set up a Thunderspike, due to its high vertical knockback.
In the Super Smash Bros. games
In Super Smash Bros., Thunderspiking is very effective, as Thunder has infinite vertical range, allowing Pikachu to garner KOs by around 20%-30% less than just an up smash would do itself.
The technique is harder to pull off in Super Smash Bros. Melee, as Thunder now strikes from a cloud at a fixed height (though it is still rather high), the falling speeds of characters are in general faster, and the move is slower and more laggy, along with Pikachu's up smash sending opponents at more of a horizontal angle in Melee, making the Thunder more difficult to setup. However, the Thunderspike is still effective on floaty characters in Melee.
In Super Smash Bros. Brawl, Thunderspiking is more effective than in Melee and it is possible to combo into it with an up tilt at much lower percentages than in the previous two games (this can be a fast and useful KO setup). Thunder can KO middleweight characters as low as 45% if properly used, making a KO way easier than all the previous games. In addition, Thunderspiking can be used during an approach at KO percentages in Brawl, as Pikachu can use a sliding up smash to set it up. However, opponents can air dodge without becoming helpless at any time (allowing them to dodge the bolt), hindering its effectiveness.
In Super Smash Bros. 4, it is still possible to Thunderspike, but due to Thunder's vertical range now being shorter, the move being harder to land in general, and the lightning bolt having a meteor smash hitbox, Thunderspiking has been significantly weakened as an early vertical KOing technique. Despite the nerfs, Pikachu can still use Thunder for other follow-ups.
The custom move Distant Thunder also has this effect on the bolt, along with having a much longer bolt to vertically KO with.