User blog comment:AYET/About the Busoshoku Haki armor/@comment-3387274-20121205131041

Busoshoku is willpower, formed into armour. You have more will than your opponent, your Busoshoku armour is tougher. that simple.

One myth I feel needs to be debunked is that it doesen't actually negate DFs. The fact that Caesar got his head punched off by Luffy, felt it, and still reformed is proof.

What it actually does is forces whatever it's imbued into to be solid. So if you use it to hit a logia user, the part of their body that's transformed still takes the hit, rather than dispersing around it. It works on Luffy for the same reason; rubber bends out the way of blunt force, and bounces it back.