How authenticated origin pulls work
Simple explanation
When visitors request content from your domain, Cloudflare first attempts to serve content from the cache. Failing that, Cloudflare sends a request — or an origin pull — back to your origin web server to get the content.
Authenticated origin pulls make sure that all of these origin pulls come from Cloudflare. Put another way, authenticated origin pulls ensure that any HTTPS requests outside of Cloudflare will not receive a response from your origin.
Detailed explanation
Cloudflare enforces authenticated origin pulls by adding an extra layer of TLS client certificate authentication when connecting between Cloudflare and the origin web server.
Standard TLS handshake
For more details, refer to What is a TLS handshake?.
TLS handshake with authenticated origin pulls
For more details, refer to the introductory blog post.