I'm referring to the scene in which Doc draws a diagram on a blackboard to explain to Marty why the two of them still exist even though they've screwed up history and created an alternate 1985. Essentially, they exist because they're in a time closure and they carry their own pasts with them into 1985-B.

It's not a perfect analogy, but it's close enough that I've been able to use that scene to help make closures click for someone who was having trouble understanding them.