Shield Mastery - Passive?

[MercuryPaladin]

Alright, what the hell is this 'physical damage vulnerability' **** on the feat Shield Mastery? I'm a long time PnP player and this means NOTHING to me.

I am assuming this is the same thing as Damage Reduction. But DR doesn't work on percentages. What exactly does this do?

And much, much more importantly... does it do it ONLY while I am blocking? I understand blocking gives extra AC from shield, but everything I've heard is 'you don't want to be blocking because you'll lose aggro', which is bad as I'm the tank.

Whether this is a straight, passive, all the time damage resistance, or some silly thing while you are wasting your time blocking, makes all the difference to me.

[Tobril]

You get the DR while merely holding the shield, so you can be attacking or whatever.

The % reduction is a nod towards DDO having monsters that hit for 100+ with a melee
attack, something not especially prevalent in pnp.

[Darknark]

Here

Yes, the new Shield Mastery and Improved Shield Mastery are passive percentage based physical damage reduction (while you're holding a shield).

[Kmnh]

Alright, what the hell is this 'physical damage vulnerability' **** on the feat Shield Mastery? I'm a long time PnP player and this means NOTHING to me.

I am assuming this is the same thing as Damage Reduction. But DR doesn't work on percentages. What exactly does this do?

And much, much more importantly... does it do it ONLY while I am blocking? I understand blocking gives extra AC from shield, but everything I've heard is 'you don't want to be blocking because you'll lose aggro', which is bad as I'm the tank.

Whether this is a straight, passive, all the time damage resistance, or some silly thing while you are wasting your time blocking, makes all the difference to me.

it's passive. If you are using a tower shield, and a mob would do 100 damage to you, it deals 75 instead (if I remember the numbers :P)