Originally Posted by
Lord Swizzlington
It seems to be a stronger version of kamui. It hasn't been fully explained, but from what I gather, Madara can basically redirect damage/swap to a shadow version of himself in "limbo", since the shadow version is invincible (according to him), he can survive fatal blows (being cut in half, stabbed, etc). Sasuke can see it due to his rinnegan, but they haven't figured out all the details about beating it yet.
That's all I've been able to scrape about it though.