When you successfully push a branch to a remote repository, you update the remote branch with changes from your local branch. When you "push a branch", Git will search for the branch's HEAD ref in the remote repository and verify that it is a direct ancestor to the branch's local HEAD ref. Once verified, Git pulls all objects (reachable from the local HEAD ref and missing from the remote repository) into the remote object database and then updates the remote HEAD ref. If the remote HEAD is not an ancestor to the local HEAD, the push fails.