It fetches all the PRs. Once you open a PR for a project (even from a fork), github creates a local branch for that PR in the upstream project. Note though tha

It fetches all the PRs. Once you open a PR for a project (even from a fork), github creates a local branch for that PR in the upstream project. Note though that doing a fetch will take some time the first time after you set up this - it will fetch all the historical PRs.