Another purpose

Chakra

With a good wind, a stable climate, and many many kilometers between them and Long Ring Island, Luffy, Zoro, Nami, Sanji, Usopp, Itachi,and Chopper were sitting in the main cabin while Robin were still resting in the next room. Itachi began to speak about his powers and skills.

"My body creates a type of life energy called chakra. It's a mixture of my physical and spiritual energy, and it's the basis of my life and my techniques. Where I'm from, chakra is essential to life, so just about everyone has it. A small portion of the population was able to generate more than normal, and as such, could manipulate it into different techniques. I am one such individual, and we called those kinds of people shinobi. Every shinobi that used chakra had an affinity to a nature transformation. The five basic chakra natures are fire, wind, lightning, earth, and water. Every shinobi has at least one form of nature transformation. For example, I have affinities for fire, wind, and water. Most would only have an affinity for one chakra nature, so to have three made me rather unique. We have a branch of chakra usage called ninjutsu. With ninjutsu, a person could do just about anything. You've seen me do it before. I can make clones of myself, spew out giant balls of fire, form massive waves, and manipulate the earth. I have thousands of similar techniques, jutsus as we call them, in my head that I have seen and copied."

He stopped to take a drink of water. Luffy and Chopper raised their hands.

He pointed at Luffy.

"That still doesn't explain how you stopped the Admiral guy. He was really strong. You didn't even touch him."

Putting the cup down, Itachi smiled. "I'm getting there."

"As Luffy said, I didn't really need to touch Aokiji to incapacitate him. The simple answer is I put him in an illusion technique, or as they are called back home, a genjutsu. It's a branch of chakra manipulation that deals with altering how one perceives the world by interrupting and manipulating the flow of chakra to their head. With Aokiji, I altered every single piece of information that his brain received, creating an illusion world where he killed me and Luffy and then took a nap. The illusion will dispel before he wakes up, and he'll continue to believe that we are dead until we do something that catches the World Government's attention. The reason I was able to cast it without touching him is because of my eyes."

He gestured to his eyes.

"I have an ocular technique, or a dojutsu, called the Sharingan. It's why my eyes are red with the three tomoe in each one. It's what allowed me to cast an illusion on Aokiji. Generally, you need to be able to touch the target before you can cast a genjutsu, but with the Sharingan, I can bypass this requirement and only need to look my target in the eyes to cast the illusion. In addition to this, the Sharingan also lets me perceive the world better than anybody else. I can pick up on the most minute of details, memorize just about anything almost instantaneously, and copy the techniques and movements of others. With it, I can observe the muscle tension in a person's body and accurately predict how they're going to move."

He stopped to let Chopper ask his question.

"You said something about mixing physical and spiritual energy. I've never heard of chakra, much less the mixing of the two energies. Could you perhaps elaborate on that?"

Itachi nodded. "Of course."

"I've been traveling for about four years, almost five now, and from what I gathered, my body is fundamentally different from your bodies. I have a completely separate system in my body called the Chakra Pathway System, which is what allows me to mix physical and spiritual energy to create chakra. Lining its pathways are 128 openings, called tenketsu, which allow me to expel chakra from my body. None of you, nor anybody that I've met so far has this system, which means that none of you can create chakra. As such, your bodies use physical and spiritual energy separately. Instead, your bodies have something different that relies purely on your spiritual energy: Haki. Haki is essentially a manifestation of your willpower."

Itachi called upon the little bit of Armament that he could do, coating his hand in Armament: Hardening.

"This is Armament Haki. This specifically is called Hardening. Armament is using one's will to protect their body or to interact with the world. It's most useful function is that it allows me to bypass Devil Fruit powers."

As he said this, he pinched Luffy's cheek and pulled. Much to everyone's surprise, Luffy cried out from the pain.

"Of course, I don't need to use Hardening for this, but it's the easiest way to show it to you. There are two other forms as well: Observation and Conqueror's Haki. Observation allows a person to read the intent of a person's actions and act accordingly to it. Quite useful in combat."

"And what about this Conqueror's Haki you were speaking about?" Asked Zoro

"Conqueror's Haki is the rarest form of Haki. Only one person in a million can obtain it as it can't be trained. You have to be born with it. It's an indicator of someone with world-altering potential. Essentially, Conqueror's allows its user to impose their will on another person's will and 'conqueror' it. This will generally cause the target to pass out or collapse." Explained Itachi

Itachi looked at Luffy, "Ace has Conqueror's Haki, but he has no control over it."

Luffy beamed, "Shishishi. Of course he does! He's my older brother!"

"If two people with considerable Conqueror's Haki were to clash, the resulting shockwave is enough to split the sky apart."

Itachi thought back to when Doflamingo had clashed with Thatch during Doflamingo's warning to Whitebeard, as well as descriptions of Whitebeard's fights with Roger.

Luffy, at that moment, realized something that Itachi had said earlier. "Wait. You're a ninja?!"

Usopp and Chopper perked up at that, looking at Itachi in anticipation. He nodded an affirmation, which was instantly met with cries of amazement from the trio. With the three gushing in front of him of how cool he was because he was a ninja, ended any further conversation.