I'm trying to understand Bond of Stone

So, I've been playing tank supports for a long time and god knows I always build a Knight's Vow and use Bond of Stone, but I never really understood this keystone mastery all the way through, so I have some questions about it:

is the 4% damage reduction applicable to true damage? is the 4% damage reduction applicable to the 6% damage redirected from allies? Is the 4% damage reduction calculated before reductions (raw damage) or after reductions (damage after defensive modifiers) is the 6% damage redirected from allies raw damage or damage after their defensive modifiers?