Official Rules

	Buckler - This weapon has an integral buckler that gives the model a cumulative +1 ARM bonus; for example, a model with two of them gains a bonus of +2 ARM. A model does not gain this bonus while the weapon system with the buckler is crippled or when resolving damage that originates in its back arc.
	Shield - This weapon is a shield that gives the model a cumulative +2 ARM bonus; for example, a model with two of them gains a bonus of +4 ARM. A model does not gain this bonus while the weapon system with the shield is crippled or when resolving damage that originates in its back arc.

Rules Clarifications

Rules Clarification : Shield and/or Buckler (Edit)

The only time you lose the Shield bonus is when the weapon is crippled, or if the attack originates in your back arc. (Infernal Ruling)
- In other words, you still get the Shield bonus while knocked down or stationary.
You don't get the Shield bonus vs stuff originating in your back arc. This includes stuff that "leaps" off models in your back arc, such as Electro Leap. (Locked thread)