[mtmail]

If I understand correctly they setup 'https://twitter-unfurl-faker.herokuapp.com/'. When twitter creates the preview it uses a Twitter user agent so the website is returning different content (or a redirect). Then a non-twitter user agent is used to access 'https://twitter-unfurl-faker.herokuapp.com/' then it redirects to https://harrydenley.com/faking-twitter-unfurling/