For normal shield bash you hold down the left button to block and tap the right button while blocking to "attack" with your shield.
For a power bash (which requires the perk) you hold down the left button to block and work the right button like you are doing a power attack with any weapon, i.e. hold it down longer instead of just tapping it.
In either case holding down the left button means your shield is raised to block, simultaneously using the right button will turn the shield into a weapon with which to attack (bash) your opponent. IF the left button is already being held down then the right button will make the shield do the striking instead of whatever weapon you might be holding in your right hand.
I would assume that, yes, a standing power attack means you are not moving when executing it. For the sprinting perk I would think you'd have to sprint rather than just run into your enemies.
I don't know about the backwards attack, I've never pulled one of those off AFAIK. If I did it was an accident.
Edit:
For a backwards power attack you have to be backing up while holding down the attack button.
Cool, I always kinda thought that was the case, I just never tried to do it on purpose.