35
I may be projecting, but I think Jon's problem was the same as mine:
I forked someone else's project (on GitHub), and needed to point the master branch of my fork to a specific tag of their project, effectively ignoring all subsequent development. (Why? After that tag, their project dropped functionality that my fork depends on and must build on. So I'm pegged to that moment in history. Sad but true.)
In this example, the tag was called 0.6.3. All I had to do was cd to my local clone (of my fork) and do
git reset --hard 0.6.3
git push --force
Then I verified on GitHub that my fork reflected the state of the code at their tag!