If you are in the Way of the Blue covenant and you get invaded by another player, someone who is in the Blue Sentinels covenant will be forcibly summoned into your world to aid you in your fight.
If you are in the Blue Sentinels covenant, you will be forcibly summoned into another players world whenever they get invaded.
Basically its a free invitation to gank. I could also see this as being a very annoying waste of time in the game, especially for speed runs. It could also ruin the game in that, depending on where you are when you get summoned, you might not spawn back there if you would die while helping the other player....
I do not know the full game mechanics, or how they fully plan to implement this idea, but it sounds like trouble no matter how you look at it. What are your thoughts?