These carry charges that can be fired to hide the vehicle behind a cloud of smoke. Once per game, after completing movement (no matter how far), a vehicle with smoke launchers can trigger them. Place cotton wool around the vehicle to show it is concealed by smoke. The vehicle may not fire and use its smoke launchers in the same turn. Any penetrating hits scored by the enemy in their next shooting phase count as glancing hits. After the enemy's turn the smoke disperses with no further effect.
Smoke Launchers
Source: Codex: Imperial Guard (1999), p. 9