When your Vanguard attacks, you do a drive check, you do this by turning over the top card of your deck and searching for a trigger symbol in the top right corner (grade 3 Vanguards can drive check twice, and G units can check 3 times). If you find one, add 5000 power to a unit of your choice, and activate another effect based on the type of trigger revealed 

Stand trigger (blue): Stand one rear guard unit, allowing it to attack again.

Critical trigger (yellow): Add one extra damage to an attack.

Draw trigger (pink): Draw one card from your deck.

Heal trigger (green): Recover one damage from your damage zone, but only if your damage is equal or greater to your opponent's.  

After a drive check is resolved, you add the trigger to hand, and your opponent does a damage check, adding the card to the damage zone and activating triggers if any