It's something that Childermass only explained to him once, and to be honest Lambert doesn't really grasp the details of it himself. All he can do is struggle back to that memory, which is a tall order considering his state of mind.

"The lives the Severing gave us ... we really lived them," he blinks, slow and hazy. "John asked the Ringmaster. That's why he and Steven became faeblooded, even after..."