By mastering one of the other combat styles, you have gained the ability to master Shield Fighting in a special capacity.
While wielding a Shield, you gain +10% Doublestrike and a +9 Combat Style Bonus to Melee Power. You gain +10 Physical Resistance Rating while using a Buckler or Small Shield. If you are using a Large Shield, this bonus increases to +15. If you are using a Tower Shield, this bonus increases to +20.
This does not work in Animal Form. This does not count as a Combat Fighting Style feat. You may not take this feat if you've taken the Shield Mastery feat.