Haoshoku Haki (Conqueror's Haki). This is a major milestone. Unlike the first time, Luffy uses this consciously and with intent to protect his friends. It shocks everyone on the battlefield, as using Conqueror's Haki consciously is a sign of a true threat.
Despite his injuries, Whitebeard smiled, realizing that the "will" he had been waiting for was present in Ace’s brother. Impact on the Story luffy using haki in marineford episode
Before this moment, Luffy was largely viewed by the World Government as a "reckless rookie". After the Haki blast, the focus of the Marines shifted; he became a high-priority target that had to be eliminated at all costs. It served as the ultimate proof that Luffy possessed the "qualities of a king," setting the stage for his post-war training with Silvers Rayleigh to master the ability. Haoshoku Haki (Conqueror's Haki)
He uses it three times in the arc (anime episodes 473, 484, and 489). Each time, the pressure wave grows stronger. But each time, it’s too little, too late. He knocks out rows of Marines only to watch Akainu punch a hole through Ace’s chest moments later. It shocks everyone on the battlefield, as using
Then, it happens.
instantly, their blades falling harmlessly from their hands.
Luffy’s will has finally reached the level of the Emperors. He has the potential to stand at the top. But potential doesn't save lives. Experience does.