Here's an example: You are England, and you get a PU over Portugal. After 50 years, you inherit them and all their Portuguese-cultured colonial nations. The Enforce Culture will change their primary culture to match yours. It will not change the cultures of any existing Portuguese colonial provinces, but any new colonies started by that CN will be your culture.What does the Enforce Culture actually do on colonial subjects? Does it actually change culture for all non-accepted cultures? Does it force convert all cultures to your primary culture? Does it factor any of your IGs or NIs to get a discount on culture conversion.
I've tried it a couple of times in a current campaign but I'm not seeing much happen.
It's also available to use on junior partners, where it might be marginally useful to gain the +5% to inheriting or an additional +1 per month integrating due to "same culture group"
Personally, I think it's rather useless, spending all that diplomatic mana for very little or no real effect.