Simple, if two users are using compatible P2P implementations, they can transfer their events via P2P, if not it goes through relays as usual. On a conversation via DMs for instance, I would assume the first DM comes in via relays and then moves to P2P when the person replies. The likelyhood of the two users being online is higher there. Then no additional DM goes through relays anymore, increasing privacy.

Of course, each user can receive the message via P2P and still send to relays for backup, but that is the receving user's choice, not the sender.