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It's the same concept as duchies drifting from one kingdom to the other, just be independent and hold those lands for 100 years and they'll eventually drift out of the old empire.
Do note that you can't drift out of an empire that hasn't been created yet.
Once the de jure years "count" hits 100, the clock starts again at zero and resumes with 1 the following year. At no point is the kingdom removed from the de jure grasp of the empire it was a part of.