This invidious spell can be cast only when the Mage has a Link of some sort active with a target, either through a Binding or through some other linking magic. The Liar's Link allows the Mage to exploit the link to weaken, deceive or persuade the target. While the original link remains active the target has a -4 penalty to Presence, Spirit or Intelligence when resisting the caster of the Liar's Link (such as resisting social skills, defending from social attacks, or resisting magical spells). The Liar's Link ends automatically when the link it relates to ends.
So long as the casting of the Liar's Link is subtle or unseen the target has little chance to realise what has been done. The Mage makes a roll of Presence + Subterfuge / Deception resisted by the target's Spirit. Only if they fail does the target sense that something about the link has changed.