The Instinct Virtue refers to a character's ability to control the Beast by familiarity rather than denial. It allows characters to "ride the wave" of frenzy and emotional excess by keeping a close rein on their passion, rather than letting passion control them. As atavistic as Conviction, Instinct is the Virtue of a monster who accepts his nature rather than maintaining a sense of human compassion.
Instinct allows the character to harness the Beast's destructive power; a character with high Instinct is primal and turbulent. A player rolls Instinct when attempting to control a character's existing frenzy. When a character who possesses the Instinct Virtue faces frenzy, she always enters the frenzy, unless the difficulty to avoid it is less than her Instinct Trait, in which case she may choose whether or not to frenzy.
Thereafter, for the duration of the frenzy, the character is wholly under the Beast's sway, and must be run by the player (or controlled by the Storyteller) in an appropriate fashion. Any time the player wishes the character to consciously take an action (known as "riding the wave"); she must roll Instinct against the difficulty to avoid the frenzy. Frenzies affect the character normally (including ignoring wound penalties, etc.), though she may sometimes exert a bit of control while so enraged.