Radio Knife is a mighty and advanced technique that Trafalgar D. Water Law wields in the popular manga and anime series One Piece. This ability is the upgrade version of Law's basic Amputate technique, thus showing his increased mastery over Ope Ope no Mi Devil Fruit powers. In Japanese, it is written as "ラジオナイフ" (Rajio Naifu) maintaining its English pronunciation.
Radio Knife is just an upgraded version of the standard Amputate. In this attack, Law charges his nodachi, Kikoku, with electricity in a manner similar to that in his Counter Shock. This electrification will give his sword even finer cutting abilities, much like how it would work with Counter Shock, only this time to create a more devastating effect on his enemies.
This unique cutting property is what differs Radio Knife from Amputate. If Amputate allows Law to cut through body parts cleanly without hurting anybody, Radio Knife takes it to the extreme. The electrical charge in the cuts makes the cuts in wavelengths, therefore very uneven. This wavelength pattern makes it nearly impossible for the victim, on their own accord—to reattach parts of their body, even within Law's "ROOM", which normally would easily allow this.
The name "Radio Knife" is not coincidental. In real life in medicine, a radio knife is a surgical instrument that cuts and cauterizes blood vessels simultaneously using high-frequency radio waves. This parallel of the fictional technique with real-world medical technology emphasizes even further Law's background as the "Surgeon of Death" and his medical knowledge integrated into his fighting style.
In the series, one of the more memorable uses of Radio Knife was during Law's fight against Trebol, one of the Donquixote Pirates' high-ranking members. Given that it actually sliced Trebol up into many pieces and prevented him from reforming his body as he had previously done whenever the standard Amputate cut into him, he showed just how effective this technique could be in this battle.
Radio Knife would definitely be one of the stronger moves by Law, usually highly placed in fan debates or official power rankings. This move's place as an upgraded version of Amputate, which paralyzes opponents, is what makes it ultra-powerful in and of itself within Law's arsenal. How this technique does more than cutting, actually preventing an enemy from recovering, increased Law's fight potential by a good amount.
It's also important to note that while Radio Knife is powerful, it eats up vast amounts of energy to perform and requires very high accuracy. This technique is usually saved by Law for either very strong opponents or for those extremely critical moments in the battle, thus appointing it high-tier status.
One of the fan favorites is Radio Knife in its anime adaptation. How many fan arts and discussions have been on social media just about the scene where Law's electrified sword slices through opponents with visible electrical currents?.
Within the One Piece world, Radio Knife represents how users of Devil Fruit become adjusted and refined to their abilities, showing how characters like Law learn to adapt powers in order to take on a rising challenge-filled world with powerful enemies as the story goes on.