The gold plate on the forehead of the high priest was tied to a hyacinth ribbon. The Targum Pseudo-Jonathan on Exodus 28:37 names the sin it was meant to repair: it make amends for boldness of face. And the Targum adds a physical detail — the plate sat on the mitre above the tephillin of the head, placing the tefillin of the high priest directly between his forehead and his crown.
Boldness of face, azut panim, is the Sages' phrase for shameless defiance. The person who lies and does not blush. The person who wrongs a neighbor and looks straight past the damage. It is, the rabbis said, one of the worst sins precisely because it destroys the mechanism by which other sins get corrected — the burn of embarrassment that makes a person reconsider.
So the high priest wore, at the top of his own forehead, an antidote. Gold is soft enough to show a scratch. A bold face is hard. By placing the metal of shame at the exact place where shame normally registers, Aaron's plate announced that in the sanctuary, faces turn soft again. Whoever came before the altar with a hardened expression left with a face capable of blushing.