Originally Posted by Omar Mendoza
Oh, that's what I figured... thanks. I just always thought it was bad, arching the back like that.
Arching is fine. OVERarching is bad (for your spine and discs, at least). Bottom line is you need to extend your hips to generate the power in a clean, not pull your arms. The arm pull needs to wait until the bar has reached past your hip crease or so.
One good way to learn proper hip extension is to practice cleans from the hang. It forces you to use explosive hip power instead of arm pull from the ground: