How do you "split" a raid ID?

By split I’m talking about going separate ways with someone who helped on the first boss in HFC. Several times now since starting this, I’ve zoned out and disbanded but when I zone in I can see the other player is killing or has killed the second boss. If I zone out and zone back in a bit later, I seem to be fine and alone in the raid. What is the exact mechanism?

Not sure but if you need help whisper me on the Game, whenever im free i help you:)