It's a pretty old exploit that people found using Otto boxes to jump straight to lvl 20 during xp bonus weekends when Harper first came out.

You need to initially level as a pure barbarian WITHOUT spending ANY ap at all until you hit level 20. Once you do that spend every single ap that you can into Harper, aka fill the whole tree, and then make sure you put Moment of Clarity on a hotbar somewhere and then reset your enhancements. When you do this it will turn into a 2nd supreme cleave once you take that actual enhancement.  There's a bug with Occult Metalline too but it requires getting all the way to 30 without spending any AP also. But if you do level all the way to 30 and then recreate the Harper Supreme Cleave Exploit and the Occult Metalline exploit you'll have a free +1 threat range while raged and a 2nd free cleave attack.

GL!

Edit: forgot to mention that hitting 30 and then doing the ap exploits also bugs out the barb rage to give extra +4str/con (friend got 6/6 somehow) over whatever you normally get.

But don't spread the knowledge less they fix it