Destructo Disk (気円斬 Kienzan) is Krillin's signature move and one of his most powerful attacks. The Destructo Disk is a razor-sharp disk of energy that can slice through nearly anything; its main weaknesses are that it is generally slow to create, it lacks accuracy, and it can't be guided once it is thrown.
Krillin first used this against Nappa, but the Saiyan dodged it (only thanks to Vegeta's advice), resulting in only a gash on his cheek. He tried again, against Great Ape Vegeta, in an attempt to cut off his tail, but Vegeta managed to jump over it. Krillin also used the Destructo Disk in a battle with the giant on Fake Namek, causing considerable damage. This giant is one of the only four characters (the others being Frieza, Cell, and Buu) to be hit directly by Krillin's Destructo Disk, but he might not count because he was only an illusion.
Later, during the battle with Frieza, Krillin was able to slice off Frieza's tail while he was in his second form as well as let loose several Destructo Disks at once. Later, he was able to throw a version that splits apart and attacks the target from multiple angles (anime only). During the Garlic Jr. Saga, Krillin used the Destructo Disk on Vinegar, who was able to catch it in his bare hand and throw it away. Krillin cut Kid Buu in half with the Destructo Disk when the Majin arrived on Grand Kai's planet in the afterlife (though Kid Buu was able to use his unique powers and immediately pull himself back together, not to anyone's surprise).
Perfect Cell using the Destructo Disk against Gohan
Vegeta, Goku, Tien Shinhan, and Android 18 have similar attacks to the Destructo Disk which they use one time each. Vegeta used it during theVegeta Saga to cut off Great Ape Gohan's tail. Goku used it in The Tree of Might, also to cut off Great Ape Gohan's tail, thus saving him from Turles' Kill Driver attack. Goku later used it to cut off Super Buu w/ Gotenks' antenna when he came to Earth in the Fusion Saga (this is Goku's first use of the move in the manga canon).
Perfect Cell used the Destructo Disk against Super Saiyan 2 Gohan as one of his last-ditch attacks to defeat him, only for Gohan to catch them mid-air and use his aura to extinguish them. Tien Shinhan used it while facing Future Trunks in the Intergalactic World Tournament in Dragon Ball Z: Bojack Unbound. Android #18 uses it when she faces Mighty Mask in the World Martial Arts Tournament to reveal him as Trunks and Goten in costume. Super Buu also can use a similar attack except his variation is red. He used this once against Goku and Vegeta before they fused in an anime filler scene: both Saiyans fired Energy Beams at Buu, but Buu countered with a volley of red Kienzan-like attacks called Mystic Shooter that cut through his two assailent's Ki beams and created hundereds of miniture blasts in the process. In Dragon Ball GT, Baby appeared to prepare a Destructo Disk attack against Uub but he was interrupted by a Ki Blast from the hero. Baby and his possessed followers are capable of performing Ki blasts that can cut people's skin easily (as shown by Baby Goten managing to cut Vegeta's face with a deflected Ki Blast). However, Krillin's Destructo Disk is moderately more potent than these and very rarely explodes if it hits a target too tough to pierce, so it can be assumed they copied it from Krillin but did not learn to use it as effectively as he is able to.
Krillin uses the Destructo Disk in Yo! Son Goku and His Friends Return!
In the manga, the Destructo Disk cannot be stopped at all (it can only be dodged) and, unlike energy blasts, can actually injure enemies many times the wielder's power level, such as being able to cut off Frieza's tail when energy blasts from the likes of Vegeta and Gohan proved ineffective. In the anime, Krillin uses it once on Perfect Cell, with the disk shattering on Cell's neck, being too tough for even Krillin's ki disk's "edge" to cut.
Baby prepares a Destructo Disk
It is interesting to note that (aside from the giant on the fake Namek, who was merely an illusion), the only ones in Dragon Ball Z whom Krillin has directly hit with a Destructo Disk are Frieza, Cell, and Kid Buu, the three main villains of the show. Krillin chops off Frieza's tail, hits Cell in the neck (which doesn't affect him), and chops Kid Buu in half. Nappa does not count, because he merely got his face scratched.
The Destructo Disk also appears in the computer game Dragon Ball Online as a skill acquired by Humans of the Spiritualist class. Unlike the original technique, the energy disk does not dismember the target; it instead passes through the opponent and this trait is substituted by the effect of bleed damage (constant damage every few seconds, over a course of time).
気 (Ki) = Energy
円 (En) = Circle
斬 (Zan) = Slice; behead; kill
Homing Destructo Disk
An improved version of the Destructo Disk called Homing Destructo Disk (called "Death Saucer" in Budokai Tenkaichi 3). It was created by Frieza, and has the ability to follow a certain target around. Krillin (as well as everyone else), were shocked when Cell unveiled this technique. Krillin even questioned whether Cell improved his attack. Cell can use both versions. Cell also used this attack in Dragon Ball Z: Budokai Tenkaichi.
The attack is supposed to be different from Krillin's Destructo Disk, but in the Super 17 Saga, Frieza uses the Destructo Disk attack against Goku in Hell, which he probably wouldn't have used in favor of his superior Death Saucer technique (however, it should be noted that he was able to control it a little better, but he still shouted "Destructo Disk!" when throwing it, hinting that he merely controlled the disk with his telekinesis)
Amond's Destructo Disk
The Crusher Corps. commander, Amond, uses an unnamed disk-launching technique where he spins on himself before launching a razor-sharp disk of energy. He used this technique to deflect Krillin's Destructo Disk in The Tree of Might.
Turtle Hermit, one of two optional classes succeeding the Spiritualist class in the computer game Dragon Ball Online, can acquire the stronger, rapid-fire variation called Consecutive Disks that is thrown in succession. This technique has area-of-effect properties ("5x8 Meter Radius Infront of and Including Single Target"), causes an additional 50% of the damage dealt if hitting the target(s) from behind, and while this version does not cause bleed damage, it stuns the opponents.
Dual Destructo Disk
A team attack used by Krillin and Android 18 in the video game Dragon Ball Z: Supersonic Warriors 2.
In the Bridge Entertainment version of The Tree of Might, Krillin refers to this attack as his "Magnetic Disks."
In the Funimation dub of Dragon Ball Kai, Krillin refers to the attack as the "Kienzan" rather than the "Destructo Disk".
In the Super 17 Saga, Goku is in Hell fighting Frieza and Cell. Frieza attempts to launch a Destructo Disk at him, and Goku is seen riding it. Frieza throws another one at him, and he balances it on the tip of a spike, so that it wouldn't touch him.