Haki is definitely One Piece's most complex powers systems, rivaling even the Devil Fruits. It can be used to, among other things, provide clairvoyance, strengthen attacks, and subjugate enemies. It has so many different uses that are still being expanded upon. For how versatile the power is, it's a must-have for anyone looking to make a name for themselves as a pirate, which is why Monkey D. Luffy trains in it.
At first, the Straw Hat Captain couldn't use Haki because he wasn't really sure what it was. This is funny as he encountered several people who used this power and he even invoked it himself a few times in his early adventures. Despite this, his limited understanding of Haki prevented him from mastering it or even knowing when it was being used. In order to excel in the New World, however, he would have to learn the secrets of Haki and master them for himself.
Haki is a form of spiritual energy that comes from a person's will or ambition. The stronger a person's aspirations, the greater their Haki. The necessity of aspiration means that while anybody can technically use it, it's not for everybody. This is why it's so rare even in the early parts of the Grand Line and doesn't become the norm until one reaches the New World.
Haki has three primary types: Observation Haki, Armament Haki, and Conqueror's Haki. Observation Haki gives the user heightened senses. It lets them detect the power, emotions, and even intentions of others. There are some who are so proficient with Observation Haki that they can see into the near future.
Armament Haki is used to strengthen the user physically. By imbuing either themselves or their weapons with Armament Haki, the user creates an armor-like coating that enhances their offensive and defensive capabilities. A more-advanced Armament Haki user can even project their Haki outward and attack without making direct contact.
Conqueror's Haki is the true tester of wills. Users with a particularly strong will can use it to make weaker-willed users submit and can even render them unconscious. This is the rarest form of Haki as only those with the greatest ambitions can wield it.
When & Where Did Luffy Learn Haki?
When Luffy realized he needed to get stronger, he took two years off from his pirate adventures to train on Rusukaina. During this time, Silvers Rayleigh taught him the basics of using Haki. By the end of his training, Luffy had become proficient in all three types of Haki.
Luffy received further training from Hyogoro. With his help, he learned to project his Armament Haki outward and destroy targets without touching them. This new application made him a worthy opponent of even the Four Emperors, especially Kaidō.
Luffy would go on to hone his Haki skills in the heat of battle. According to Rayleigh, intense fights are the best place for his Haki to truly blossom. Against Katakuri, he learned to use Observation Haki to anticipate attacks. Against Kaido, he learned to project his Conqueror's Haki to cause further damage to the Emperor.
As of Chapter 1042, this is the extent of Luffy's Haki abilities. He could still learn more applications later down the line. With that said, splitting the sky by clashing Conqueror's Haki is pretty impressive even by One Piece standards.
