When activated, the Invisibility Watch allows the Spy to become invisible to the enemy team for ten seconds. If the Spy takes damage or brushes an enemy player they will flicker briefly, revealing a team-colored silhouette.
The Spy is unable to attack while cloaked and there is a slight delay between decloaking and being able to attack. However, Spies are able to use their Disguise Kit when cloaked, enabling them to switch which member of the enemy team they are disguised as if their cover is blown.
After a full ten seconds of cloaking, the Invisibility Watch takes thirty seconds to regain a full charge. However, players do not have to wait the full thirty seconds, instead taking a penalty to cloak duration. The cloak meter may also be partially filled by picking up ammo boxes and dropped weapons. A skilled Spy can navigate an entire map completely invisible by moving from ammo box to ammo box.
The Spy's greatest foe is the Pyro who can light the Spy on fire, effectively nullifying the usefulness of the Invisibility Watch as the enemy team is able to see the Spy's burning silhouette.