git rm will remove entries from the staging area. This is a bit different from git reset HEAD which "unstages" files. By "unstage" I mean it reverts the staging area to what was there before we started modifying things. git rm on the other hand just kicks the file off the stage entirely, so that it's not included in the next commit snapshot, thereby effectively deleting it.
By default, a git rm file will remove the file from the staging area entirely and also off your disk > (the working directory). To leave the file in the working directory, you can use git rm --cached.
But what exactly is the difference between git rm --cached asd and git reset head -- asd?