They work great. Much better than the old method.
They convert provinces around them. You can see the progress bar of conversion with all modifiers that change the speed. They prefer to convert provinces in countries with state religion that matches religion of the center. They will target random provinces, but the closer to the center, the more likely it will be picked.
For example, I was playing as Brandenburg and converted to Protestantism. This created Center in one of my provinces. Bohemia also converted, so I had two centers of my religon and soon both of them were busy converting my provinces. Few years, and my whole country is Protestant. Then they began converting catholic provinces in other countries.
Instead of randomly dispersed provinces, now conversions make sense and whole, continuous regions are converted.