GEAR SECOND
This technique involves Luffy speeding up the blood flow in all or selected body parts, in order to provide them with more oxygen and nutrients thus making him much faster and stronger. The technique uses even more oxygen and energy, using up more food and causing Luffy to pant heavily.
Luffy corks his mouth with his thumb, bites into his thumb joint (making a small opening) and blows very hard into it, which inflates his arm. He is then able to transfer the air through his entire body, requiring the torso to act as an interface for his various limbs. Luffy blows so much air into his body that when they are all focused on a single limb, they become the size equivalent to (or greater than) that of a giant's, if anything resembling Oars size.
Haki (覇気 Haki?, literally meaning "Ambition") is a mysterious power that is found in every living being in the world. It is not that different from the normal senses. However, most people do not notice it or fail to awaken it. Broadly, there are two types of Haki common to everyone, given the proper training; however, there is a third type that only a certain group of "chosen ones" are said to possess.[1][2] In simple terms, Haki is an ability to sense spiritual energy and overpower enemies.