It seems that back in Alabasta, Zoro was feeling the breath in everything: http://www.mangapanda.com/one-piece/195/16
http://www.mangapanda.com/one-piece/195/17
And thanks to this, he could decide what to cut and not cut, going from not cutting leaf to cut rock and even steel: http://www.mangapanda.com/one-piece/195/18
Curiously though, i don't remember if it has been explained in the manga again(not sure). So what do you think it was at that point?
http://www.mangapanda.com/one-piece/195/17
And thanks to this, he could decide what to cut and not cut, going from not cutting leaf to cut rock and even steel: http://www.mangapanda.com/one-piece/195/18
Curiously though, i don't remember if it has been explained in the manga again(not sure). So what do you think it was at that point?