This is a advance way to use the kunai in its advance form and is known as an explosive kunai (起爆クナイ, kibaku kunai) only created by attaching an explosive tag to the blade either by wrapping the tag or by having a string inside the ring at end with a tag at the end. The explosive kunai is usually thrown so that it lodges in an object near the target, and then the explosion from the attached tag destroys the target.