git push remote local_branch:remote_branch
An example of when to use this is in combination with Heroku. You might want to push your local branch to Heroku's master branch:
git push heroku foo_branch:master
That is also how you make Heroku run a non-master git branch.
Maybe you want to push the contents of your current branch to another branch on git:
git push origin foo_branch:bar_branch