Caution! Do not make this at home!
The knife works by using a powerful spring to launch the blade about 20 feet. When combined with the standard, overhand knife throwing motion, the power behind the weapon is very great and would likely pierce soft body armor.
To fire, one would remove the pin (like on a grenade) and depress the firing button.
Spring-powered ballistic knives briefly gained notoriety in the United States in the mid-1980s after commercial examples were marketed and sold in the United States and other Western countries. Since then, the marketing and sale of ballistic knives to civilians has been restricted or prohibited by law in several countries.
Similar to conventional automatic knives, federal law makes ballistic knives with a spring-operated blade illegal to possess, manufacture, sell, or import “in or affecting interstate commerce.” This means they are illegal to import from outside the United States, as well as buy or sell over state lines, including possessing or making them with intent to sell over state lines. The federal law also makes it a separate crime to use or possess a ballistic knife during the commission of a federal crime of violence, with a minimum sentence of 5 years in a federal prison.