From the git man pages 
> git push origin master:refs/heads/experimental
Create the branch experimental in the origin repository by copying the current master branch. This form is only needed to create a new branch or tag in the remote repository when the local name and the remote name are different; otherwise, the ref name on its own will work.
So, a direct substitution of `git push origin :experimental` says
Create the branch :experimental in the origin repository by copying no branch
PUT a branch refb with the commit-ish refa
PUT a branch refb with the commit-ish abc123
PUT a branch refa with the commit-ish null