Basically, your character is temporarily replaced by a character from their own canon who has some sort of important connection to them. You're allowed have characters disappear at different points during the flood so the character they're replaced with can actually talk to their CR and leave messages with them for the character and so on.

It's not something that works for every character, but it's great for obscure characters who are unlikely to ever get that castmate, and I find it can be used for a lot of things.