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The user flows chakra through all parts of their body causing it to become noticeably darker, which increases their defensive power to its utmost limits by making the skin as hard as diamond, easily capable of withstanding most attacks with little to no damage. As a result, the amount of techniques that can break through the areas hardened by the Earth Spear are close to zero, excluding its undeniable weak point: Lightning Release ninjutsu. Furthermore, the destructive power of physical attacks is increased, making this a great all-purpose technique.
Kakuzu demonstrated the ability to use this to cover himself partially (i.e. his hand and lower arm, and the side of his head) without using hand seals, which gave him a significant tactical advantage against ambushes.