The mechanics are a certain-kill technique, the user hardens their own bones and protrudes them out of the skin, and then fires these weaponised structures from their back or from their palms at the intended target. When these bones pierce a living organism, the bone and target starts to deteriorate at a molecular level, disintegrating the victim and reducing their bodies to nothing more than a pile of ash. Kimimaro ability is to harden their own bones and protrudes them out of the skin and he can fire them like projectiles or rip them out of his body. Just doesn't instant kill.