Friendly rebels: how do they work?

[Gnostek]

Picture this not-so-uncommon situation: you're playing Manchu, and Ming gets manchu rebels. They start sieging down provinces, and after 5 years, these provinces defect to you. Now, some of these provinces might be cores, while others will not. I'd love to know a way to determine how this works. Thanks a bunch!

 

[inreadible]

I'm not a 100% sure on this but I think rebels make cores in your primary culture provinces they siege.

edit: From Eu4wiki:
"If separatist rebels win a siege, they extend separatism by 10 years (+5 "Separatism" local unrest modifier) and place a core of the primary nation of that province's culture if it does not exist there already"