It gives you one extra talent point for each 'talent' effect it applies to you. I had 3 of them equipped and it gave me 3 talent points every time I left the mirror

This happens when someone has an item equipped that gives a talent that they don't meet the prerequisites for.

A friend of mine was wearing the (Grunt Helmet?) which gives the Headstrong talent that requires Warfare 5. If he respecs and doesn't meet the prerequisite of the talent he'll leave the mirror with a bonus talent point while still having the talent. This is easily reproducible with any item that provides a free talent but doesn't cover the prerequisites of the talent (off the top of my head: Unique Warrior pants, Grunt Helmet, Winged Boots, Ryker Belt)