The Progressive Knife (also referred to as a Prog Knife), it is the standard-issue close-quarters combat weapon designed specifically for use by the Evangelion Units. It is stored in the shoulder casing until ejected by the pilot.
Unlike an ordinary knife which tears into an object with the brute force of its sharp blade, a Progressive Knife uses a blade of high-frequency oscillating particles, allowing it to cut matter on a molecular level. This is also the reason the blade radiates a certain hue. Since Evangelion Unit-02, a box cutter-type knife was adopted, allowing the knife to be used even once damaged by further retracting the blade.
Based on Neon Genesis Evangelion Platinum: 06 DVD Booklet