User blog comment:Traffy731/luffy's red hawk attack/@comment-75.31.0.164-20120729130135

Actually, based on what I've seen, the ability is possible with Luffy's Gomu Gomu ability if you consider the properties of how his Haki effects his body. I certainly find this explaination a lot more likely then he found Ace's Devil Fruit, since he never showed any ability to turn into fire up to this point.

Luffy activities Gear 2nd and uses Bokushoku Haki for Red Hook, correct? Bokushoku Haki seems to Vulcanize his rubber body, making it more like tire rubber. Gear 2nd is also shown to produce a lot of heat, as shown from the steam coming off of him when he normally uses it. If the two are used together, the fissure of his blood moving superfast, brushing up against the now firm rubber, combined with the heat he seems to generate anyway, could make enough heat to ignite the rubber and cause it to burn. And the mere speed/force of the attack can cause the explosion, though the fire probably helps too.

Now, that being said, I certainly do think that he thinks of his brother, "Fire Fist" Ace, when he uses the technique... he probably developed it in his honor to begin with.