I watched a youtube video on how to use Auriel's shield and then tried it out myself, but I couldn't get it to work. I let a cave bear power attack me for a while until the shield was glowing/flaming and then held down the bash button, but nothing spectacular happened. Maybe I just don't know how to power bash (I do have the perk). Isn't power bashing where you hold down the left button to block, then hold down the right button to power attack? What am I doing wrong?

Doesn't sound like anything. The shield has 3 different stages of being charged though, so make sure it's all the way there before you bash.

power bash= hold your shield raised and HOLD IN your attack button.

Otherwise you'll do a normal bash