How does Github.com know to refetch its Pull Request page when a commit is pushed

On Github.com when you view a pull request you can see a timeline of all the commits that belong to that branch.

When a new commit is pushed to that branch - the page is automatically refetched.

We can see this by looking at the network tab a GET to https://github.com/_render_node/MDExOlB1bGxSZXF1ZXN0NTA1MzExMTky/pull_requests/body

Is sent, and this returns the new HTML with the new commit.

• They do not use WebSockets
• There is no poll request

So my question is how do they do this?