User blog comment:Traffy731/luffy's red hawk attack/@comment-1405658-20120725192759

I almost thought that could come up far later. The important detail on how Blackbeard got Whitebeard's power involved his corpse, almost immediate prior death, and an undisclosed method obscured by or maybe involving a cloth/tarp. Now, Ace pretty much died in Luffy's arms, so you have the possibility of "absorption" like blackbeard did. If it was going to happen, it'd be then and there. It might even be repressed somehow, if the absorption happened while Luffy went catatonic.

As explained and even commented earlier on, a person who eats 2 devil fruits will die. But, there IS that little exception involving a devil fruit user dying first, and Blackbeard and his crew are proof of that, judging by the rumors that his crew are hunting devil fruit users to absorb their powers (Chapter 650, page 8).

Given Tashigi's difficulty controlling Smoker's powers in his body, there's the outside chance that Luffy doesn't know how to activate it yet if he even has that power.

Most likely, it's a combination of Haki and speed. (we could roll it into Haki, notice how Sanji literally burns when he gets emotional? [ie: a chance to cook a deep-sea fish, Chapter 654, page 6]).