About half a lifetime ago I spent a summer building decks and sunrooms, so we had plenty of opportunities to screw up operating a two-person power auger. As others have said, know which way the handle is going to move when you hit something solid and be ready for it. You might think you're being smart by bracing the handle against your thigh but you'll change your mind when it the bit grabs on a rock or a root. Much better to have the handle jump out of your hands and spin than to add black and blue decorations to your body.
You mentioned working in clay, so make sure you take the smallest of bites. Basically work to hold the auger up the whole time and barely let the tip make contact with the ground, clearing the hole often. You want to pull dirt up, not drive the auger down. Once we were working on a site with lots of clay and weren't paying enough attention. The auger got a hold and screwed itself straight down until the handles were flush with the ground (with our fingers underneath). Happened in a flash, suddenly we were just pulled down to our knees. Obviously there was no lifting it out of that hole, so the next while was spent with a 10' 2x4 jammed through the handles walking in circles like Conan to screw it out. Not the best use of anyone's time!