Well lets say youre Swiss culture (for example, lets say were not talking about Vanilla).
One of the territories neighboring you is also Swiss culture, BUT is owned by another country with different primary_culture, say Austria, Austrian culture.
If youre the ONLY Swiss cultured country, IF you use spies to spawn patriot rebels in Austria-owned Swiss province, they will besiege it and defect it to you after 32 months (or something like that). IF there is another province of Swiss culture, REGARDLESS of who owns it (it can even be another Swiss cultured country!!!) neighboring the one where you spawned patriot rebels via spies, the patriot rebel stack can upon successful siege simply move to that one and do the same thing (or if Austria brings a doomstack that defeats the patriot rebels, they will RETREAT to ANY neighboring Swiss province regardless of who owns it): besiege and conquer it. If there are even more Swiss provinces, not owned by YOU, rebels will keep moving around besieging them one after another, after some time you will start getting provinces defected notification
The HRE area is specially good, because there is alot of small one province countries. Usually, if rebel stack is spawned in high base_tax province, rebel stack can be even 10-15000 strong. In most cases, as soon as a rebel stack of that size moves to a OPM country, it will insta-annihilate OPM's army (usually kept at low maintenance) and proceed to besiege it. IF OPM's capital is taken by patriot rebels, rebels will enforce their demands immediately, or in other words DEFECT the province right away.