If you are facing your enemy and if the ememy hit you, you'll have a chance to block it.

If your side is facing the enemy and if the ememy hit you, you'll have a less chance to block it

If your back is facing the enemy and if the enemy hit you, i THINK you DEFINATELY get it.

Unless, of course, you have blade grasp or something like that.

I would show you a screenshot but anyway...
There is a percentile somewhere...heck, it's hard to explain, I'll go look for the screenshot.