An army's vanguard troops advance to engage the foe before their comrades. Sometimes this is a point of honour, sometimes because they are eager to fight the foe and occasionally because they are expendable, and their general wishes to tire the enemy out.
After both sides have deployed all their other forces (including), but before the roll to see who gets the first turn is made, units with the Vanguard special rule can immediately make a move up to 6" (12" in the case of ) move regardless of their Movement value (they cannot march and are affected by terrain as normal). This cannot be used to move the Vanguard troops within 12" of the enemy. A Vanguard move does not count as moving for the purpose of shooting in the first turn.
If both players' armies contain Vanguard troops, players should roll off. The players then alternate moving their Vanguard units one at a time, starting with the player who won the roll-off.
Units that have made a Vanguard move cannot declare a charge in the first turn if their army goes first. This also includes charging in the Magic phase.
If a character is deployed as part of a unit with the Vanguard special rule, that unit can still make its Vanguard move if the character in question has the Vanguard special rule or is a model on foot. Otherwise the character's presence prevents the unit from using Vanguard.