Using this option effectively creates new commit! It replaces current tip of the branch, so if you use it on anything that's NOT the tip (checking out a tag or SHA and then amending) you're effectively branching (feel free to check with logs).
It's considered a rough equivalent of
- soft reset,
- manual changes
- and committing on top of ORIG_HEAD.
- You may also use it to amend a merge commit.
--amendyou also reset author and author timestamp.
git help commit