Not sure if I understand your question, but I´ll answer as well as I can: 

A spirit/entity has been infused into his sword, and it moves the sword to block incoming projectiles for him, because he would not be able to block things like bullets himself, but it can only do so when wielded. What I mean by that is that if he´s holding the sword in his hand, and a bullet or something is coming towards him, the sword will move with the hand over to deflect the bullet, whether he wants to or not. 

Edit: It does not do that against melee attacks.

Edit Edit: Also, your signature, what is it from? That is one cool looking fight.