Here's a bit of clarification on the Shield Mastery Protection talent (the notes were just updated to reflect this): http://us.battle.net/wow/en/blog/2356431
The specific mechanic is that if you have the Shield Mastery talent, then your Shield Block procs a second effect called Spell Block. Spell Block reduces wow po magic damage by 7/14/20% for 6 seconds.
We also just added new notes for the Strikes of Opportunity (Arms) and Unshackled Fury (Fury) mastery bonuses, so be sure to check those out.
/https%3A%2F%2Fassets.over-blog.com%2Ft%2Fcedistic%2Fcamera.png)