Pressing the assigned button toggles on/off the medallion. Holding the button for a couple seconds while you’re using it creates a shell of yourself that is left behind in the spot you created it. The shell dissipates when you press the action button or if you stop using the medallion. The shell can be used to hold down switches, and perhaps other uses. The shell can be destroyed when exposed to direct beams of light, from the sun or light type attacks.
While using the medallion, illusions become obvious. Spikes or blocks that aren't real will have some kind of visual effect, and will be passable while the shadow medallion is used. Some invisible paths will become visible as well. Using the medallion consumes your magic.