In the chaotic climax of the Marineford Arc, Monkey D. Luffy is far from the refined Haki user he becomes post-timeskip. He has no control over his abilities, and he isn’t even aware of what Haki truly is. Yet, despite his inexperience, Luffy unconsciously uses two forms of Haki during the battle — moments that are easy to miss but deeply significant for the story. The most famous instance occurs when Luffy saves Ace from the execution platform. As he stands between the two admirals and countless Marine officers, Luffy lets out a powerful scream of desperation and will. In that instant, he unleashes a burst of Conqueror’s Haki, knocking out a significant number of nearby Marine soldiers.
In short, Marineford is where Luffy’s Haki first awakens — but it’s Sabaody and Amazon Lily where he learns what that awakening truly means. when luffy use haki in marineford
Here’s a well-developed text on the subject: In the chaotic climax of the Marineford Arc, Monkey D