Unlocking dead screen clarification

Hi All,
I´m just curios..I read a comment on some site about how some users are trying to unlock their phone´s with a dead screen and they were saying that everything seemed to be working fine but that they can´t see the mouse pointer on the screen of their PC. They said something about using an HDMI adapter. I have also read that in order to mirror your phone to your PC it has to be unlocked or else it won´t work, so these people are basically wasting their time, right? Thanks.