I have used a clam-shell post hole digger most of my life, and i didn't know this tool existed. OMG! So much easier to dig a fence-post hole. It works like a corkscrew or a drill bit. Just press on it and crank it around. Works in clay and handles small roots up to 1/2" or so. I had trouble with some rocks in a couple holes, but who doesn't?  Great uniform 6" holes!