gitlab: invocation of gitlab-shell

I have been going through the code of gitlab-shell. I am unable to understand how it is invoked. If I made a pull to my server(on which gitlab-shell is installed), how does gitlab-shell knows about it? bin/gitlab-shell has this line as comment:

GitLab shell, invoked from ~/.ssh/authorized_keys

Is there any line in authorized_keys which invokes gitlab-shell? (sounds unlikely).

Git Baby is a git and github fan, let's start git clone.