Ashura is way और powerful. tht's why he used it against Kaku. It's a form of haki,which is a special technique only a few people can do,luffy and shanks share this ability.shown first when shanks shooed away the sea king trying to eat him and luffy,and also when luffy and bonchan r in impel down and luffy blasts away all those wolves.so its kinda like going gear third in a way,but for zorro.