I think he talking about the metal button that attaches the mirror to the windshield. I assume, he is trying to remove one that it is still in the replacement mirror, or trying to remove his old mirror from the windshield.
The service manual says to remove the plastic cover at the base of the mirror and position the mirror in the full down position, then lift it up and off of the button.
I guess that positioning the mirror all the way down must move the locking tab out of the way of the mounting button.