This is a great question. My friends and I often wonder about this. The Reunion is the one song where it's obvious he does it. He might do it on the Our House verse when he comes with the faster flow near the end of his verse. I would like to see other examples of this as well.
This is niave as fuck, but i like to think he just hops in the studio and one takes it like Jay does. I picture him doing that with No Love and think its incredible, but obviously that can't be the case. He is a perfectionist and probably does each verse numerous times over.
Anyway, I like this thread and can't wait to see where people think he does this.