When used, Snake bends down and plants a C4 on the ground where he is standing. By using the move a second time, he takes out a remote with a flourish and exclaims "Now!" (or "There!" in Ultimate) as he presses the button, detonating the explosive, which does 17% damage and can KO at medium-high percentages. The detonation can be cancelled if Snake is attacked before he presses the button.
Should Snake be close to and facing a wall, using the C4 will stick it to the wall instead of the ground. If used in midair, Snake will drop the C4 with a less obvious animation, which falls straight down and lands on the ground below. Should he lose his Cypher to the grab release glitch, Snake can fast-fall to catch up to the dropping C4 and bomb jump to regain his recovery. If Snake is standing near an opponent, he can attach the C4 directly to them. The C4 then acts as a Gooey Bomb, able to transfer between players should they pass by each other; the small dark green box is much harder to see than an active Gooey Bomb, so Snake must be careful it has not migrated back to himself before he blows it up. C4 stuck on characters will fall off after 15 seconds and can also fall off if the character takes a strong blow. In Ultimate, the C4 has a red light on it that will blink for three seconds once attached, increasing its visibility. While the light blinks, it cannot attach to other players. The light will blink again if the C4 attaches to another player.
If Snake chooses not to detonate the C4, it will explode automatically after 26 2/3 seconds. Unlike his down smash in Brawl, the C4 does not explode automatically when Snake is KOed, which can allow Snake to potentially KO opponents even when he has no stock remaining.
There is a 1-in-6 chance that Snake will plant a butterfly-shaped C3 explosive instead of the regular brick-shaped C4. This easter egg only changes the explosive's shape; its damage, knockback, and other properties do not change.
The Omnigay, also known as The Omni, is a technique that takes advantage of both the C4 and Faux Super Armor in doubles play. Named after the smasher Omni, it is performed by attaching a C4 on a Meta Knight or Kirby, after which the character must grab an opponent with a lower port, use their up throw, and while the two are above the upper blast line, have Snake detonate the C4. If done properly, the player with the lower port will be KOed above the upper blast line, but the player with the higher port (the Meta Knight or Kirby) will usually receive no knockback and survive due to Faux Super Armor. In general, the C4 is applied to Snake's teammate; while the Omnigay does work if Snake places the C4 on an opponent, doing so is impractical, due to the attack's short range and Snake's inability to combo into it.
The Omnigay is most effective when Meta Knight or Kirby is player 4, as this allows the player to gain Faux Super Armor over any opponents; as player 2 or 3, the Meta Knight or Kirby can only perform the Omnigay on player 1. Owing to this, tournaments generally require players in Brawl doubles to agree upon ports, often via rock-paper-scissors prior to the match; furthermore, some rulesets may require players to switch controller ports following matches, depending on the victor.
While the Omnigay can be devastating, it is difficult to set up, as it requires Meta Knight or Kirby to grab the opponent before the C4 automatically explodes and it also requires that Snake can safely detonate the C4. The Omnigay can also unexpectedly fail and KO both Snake's teammate and the opponent they are throwing, regardless of the potential faux super armor. The exact reason for why the Omnigay sometimes fails is not known; it has been theorised it could be due to the placement of the C4 in relation to the characters, incorrect timing of the detonation, or the technique having some sort of random chance to not work as a result of its various elements.
A variant to the Omnigay can be performed in singles, where a Meta Knight or Kirby player can grab a Snake holding onto a Hand Grenade, then use an up throw so that the grenade detonates above the upper blast line. This variant, however, is extremely difficult to perform, due to it requiring precise timing and knowledge of when the grenade is about to explode.
Composition C-4 is one of the most common forms of plastic explosive (aka plastique) in the world. Like all plastic explosives, the C4 has the consistency of clay, allowing it to be molded into any shape and is incapable of being detonated by anything other than the detonator itself, making it safer. Unlike other plastic explosives, such as the C3, a C4 retains its clay-like consistency in all conditions. These properties have led to it being used worldwide, particularly in the US Army. The C4 is a recurring item in the Metal Gear series, and it is always required to progress through the game, particularly as the item required to destroy the TX-55 Metal Gear in the first game.
The C3 butterfly is from Metal Gear Solid 3: Snake Eater. In one cutscene, Naked Snake (Big Boss) models a C3 pack into a butterfly, throws it into in the air, catches it, and exclaims "Gotcha this time!" before slamming it against one of the Shagohod's liquid fuel tanks; the scene references an earlier event in the game where Snake is unable to grab an actual butterfly out of the air due to a lack of depth perception after losing his right eye.