ILuffy has developed all his gears based around the rubbery properties of his Devil Fruit. Gear Second makes use of his rubbery body to pump blood faster through his body and increase his physical abilities drastically. Gear Third sees him inflate his rubbery bones with air to gain the limbs of a giant and use devastating attacks. With Gear Fourth, Luffy takes things a step further by incorporating Haki with his Devil Fruit and combining them together. By doing so, Luffy is able to increase the tension of rubber and use the power of compression and elasticity in combat to pull off some of the most incredible things such as flying through the air