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