To render oneself completely invisible except their eyes.
Sheer force of will (presumably)
Voyeurism is a variant form of Invisibility that renders the user completely invisible except for his or her eyes, which glowed as the power is being used. Because of this one flaw, users must position themselves in locations where the visibility of their eyes are at most minimal. It is commonly used to spy on others. It is also related to the teleportation power of Shimmering.
The demon Klea was often employed to use her voyeurism power to gather intelligence for her compatriots in the Brotherhood of the Thorn. Her eyes remained visible for about a second whenever she shimmered out. When Cole pretended to return to the Brotherhood, the others didn't trust him and often dispatched Klea to find out if he was actually spying on them.
When Vornac was vanquished by the Charmed Ones, Cole appeared to them and told them he had to make them think he'd turned. Unknown to everyone, Klea was spying on them, and immediately alerted Raynor that Cole was working hand in glove with the Charmed Ones.