A way of saving a non-magical being from attack that can only be performed on a non-magical being. As seen in the chapter. The mix of magic blood acts like a ward and a cure. Though this is not always fool proof and is very dangerous.
Wouldn’t recommend the use, it binds the person to the blood giver with the magic, it often transfers the magic into the person which can accelerate mental instability. But on some occasions it can have a great positive effect.
Oh also did I mention don’t try it, blood borne diseases and all that.