[jzila]

Great question!

We give a little bit of detail here: https://keybase.io/docs/kbfs#frictionless-sharing

But the basic model is that when you share or chat with someone@twitter, that content is only encrypted to your devices. When that Twitter account posts a proof, and announces that proof on a Keybase account, Keybase's servers will notify your devices including a link to the tweet. Your device will independently verify that the cryptographic proof is validated by the keys of the Keybase account claiming it, after which it'll re-encrypt the keys to that data for the newly verified Keybase account.

This all happens seamlessly in the background.