A "kata" is the pattern of movements one makes when learning a new hand-to-hand combat form. It's pretty important to understand the flow of the movements and commit it to muscle memory before you even begin working with something like a kunai. For example...

[ Kakashi goes through a slow series of hand movements, gracefully cutting through the air with his palms. ]

This series of kata can be very effective on its own. But with a kunai...

[ He pulls out a kunai and demonstrates -- this time, the blade cuts through the air so swiftly there's a shhh of sound as it passes. It would be very easy to see how it is as lethal as it is graceful. ]

The attack changes from crushing injuries to potentially lethal ones.