In traditional Chinese culture there is a belief in something called Chi (or Qi), usually translated to energy flow, noteable the literal translation is "
breath". I'm no expert but from what I can recall Chi is life force inside every living being, and if you train yourself to use it you can accomplish feat you couldn't do without it. Such as spear trust to the throat and survive, even breaking the spear, standing on one finger, lifting up your whole body etc. So roughly Chi is able to enhance you if you so use it.
Roronoa Zoro is known for his religious referrence, Asura for example which is a power-seeking deities, sometimes referred to as demons in Hinduism. Kaku noted that Zoro used his spirit to enhance himself and that Asura was a manifesting of his spirit. When Zoro fought Mr. 1 and he reached the brink of death, he said he was able to "hear the
breath of all things", making him able to not being able to cut a leaf while he could cut steel if he choosed to.
My theory is that Roronoa Zoro isn't using Haki, but rather his inner life force (his spirit); his Chi, when he is manifesting Asura.
My impression of Haki is that you use a spirit rush that manifesting outside, on others, not on yourself. Of course we have not seen much of it yet so I may be totally of the mark. marvelB's theory that you can use a Haki rush to disrupt a Logia user into being solid sounds very probably to me, and I hope that it's a use we will see (knowing Oda, either way I atleast wont be disepointed even if it doesn't turn out that way
).