As best as I can understand is that the pain is an optional bonus, but the leverage is the key and they should go down regardless of whether it hurts or not. There are always a few freaks* who never seem to feel pain no matter who does yonkyo on them. But they still get their faces driven into the mat if someone knows how to do it well.
* I mean 'freak' in the nicest, friendliest, least insulting way possible
.