Spike

A spike is an attack that sends the opponent downwards. Unlike a meteor smash, a spike cannot have its knockback interrupted via meteor cancelling, making it an extremely dangerous tool for edgeguarding offstage.

In Super Smash Flash
Although the How to Play tutorial mentions meteor smashes, there is no meteor cancelling in this game. Thus, all downwards-hitting attacks are spikes. Performing a spike requires that the attacking player hits the opponent with a move that deals vertical knockback while the attacker is positioned above their opponent. Due to the presence of the Instant Kill Glitch, which affects all vertical knockback, spikes are generally not particularly useful unless the opponent is not moving at a speed fast enough to score a horizontal KO with.

In Super Smash Flash 2
As of Version 1.2.3, a downwards-hitting attack must have a base knockback angle lower than 250° or higher than 290° to register as a spike. Some moves possess special knockback properties that turn them into spikes under the right circumstances, most notably multi-hit attacks that use autolink angles, including 's and ' neutral airs. Because meteor cancelling can only be done while in the "sent flying" state, some meteor smashes work like spikes at low percents if their knockback is low enough to not trigger that state.

List of spikes by character
The following list only includes moves that naturally have the ability to spike without using knockback stacking or other unorthodox situations.

Trivia

 * Prior to Version 1.3, moves such as 's down smash could spike opponents due to quirks with the knockback stacking mechanics. In this case, it's due to combining the knockback vectors of the meteor smash hit with the semi-spike hit, resulting in a knockback angle outside of the meteor smash range.
 * In Version 1.2.3, Marth's tippered down aerial, despite technically being a spike, did not work properly because it was programmed to immediately force characters into tumble fall, thus allowing them to jump out of it early. This behavior was fixed in Version 1.2.4.