The psy-snare was a piece of technology available to the Sontarans since the early times of the Time Lord civilisation. It was a "brain wiper", able to erase the mind of its victim and reprogram it. The device was disc-shaped, to be applied to the temple or the forehead of a humanoid head.
On Desrault, the site of a trilateral negotiation, the Sontaran ambassador Brilox used it on Millenium, member of the Gallifreyan delegation, and ordered her to kill the Elder of the Black Sun. In revenge, Wardog used it on Brilox himself. (COMIC: Black Sun Rising)