So it always updates the HEAD pointer, except when it doesn't, and you give it the name of an existing branch that represents the commit to change your working tree into, except when you specify -bf to use the HEAD commit, and except when you specify -b and a branch name that doesn't exist. You can also specify start_point to use instead of HEAD.

So, the checked out commit is the second ref specified, or HEAD, unless the flags are such that you can only specify one ref.

Git checkout also "merges" uncommitted changes with your new HEAD, which you didn't mention and makes me wonder why the command isn't called "git merge".