Two things to make sense of how origin/HEAD is working.
First is that HEAD always points to the commit that you currently have checked out. So, for example, if you have the master branch checked out, HEAD will be whatever is the latest commit in master in your local copy of the repository.
Second is that to git there is no difference between the originating repository and your own. If you had your copy of the repo fully exposed, the remote machine that you cloned it from could push changes to your copy exactly the same way that you push changes to the remote machine.
That second part is important because it means that the remote machine you're pulling from also has its own HEAD pointer. When you reset your state to origin/HEAD, you're telling git to set your own HEAD to point to the same thing as the remote machine's HEAD. This is very likely to be the same asking it to reset to origin/master because HEAD is probably pointing to master on the remote machine.
The reason it's likely that HEAD=master is that in most circumstances the repo on the remote machine isn't manually being touched. When you first setup a repo there is only a single branch so that is what HEAD points to. If no one ever logs into the machine and executes a checkout command, that's what the server is going to continue to point to.
However since there's no guarantee that HEAD=master, you shouldn't rely on that and instead always use the actual branch name.
Thank you, that clears it up! And probably explains why I've lost work when resetting other branches; I wanted to revert to the latest remote tip of their branch, not reset that branch to master, but I screwed up because of HEAD. Thank you for clarifying!
First is that HEAD always points to the commit that you currently have checked out. So, for example, if you have the master branch checked out, HEAD will be whatever is the latest commit in master in your local copy of the repository.
Second is that to git there is no difference between the originating repository and your own. If you had your copy of the repo fully exposed, the remote machine that you cloned it from could push changes to your copy exactly the same way that you push changes to the remote machine.
That second part is important because it means that the remote machine you're pulling from also has its own HEAD pointer. When you reset your state to origin/HEAD, you're telling git to set your own HEAD to point to the same thing as the remote machine's HEAD. This is very likely to be the same asking it to reset to origin/master because HEAD is probably pointing to master on the remote machine.
The reason it's likely that HEAD=master is that in most circumstances the repo on the remote machine isn't manually being touched. When you first setup a repo there is only a single branch so that is what HEAD points to. If no one ever logs into the machine and executes a checkout command, that's what the server is going to continue to point to.
However since there's no guarantee that HEAD=master, you shouldn't rely on that and instead always use the actual branch name.