A trooper or ground vehicle unit can perform the standby action to gain a standby token. Place it on the battlefield near the unit’s unit leader. A unit cannot perform an attack action and a standby action in the same activation. A unit can only have one standby token at a time.
During the Activation Phase, after an enemy unit at 
and in LOS of any miniature in a friendly trooper or friendly ground vehicle unit with a standby token performs an attack, move, or action, if it is not that friendly unit’s activation and that attack, move, or action was not caused by an effect of that friendly unit’s controlling player, that friendly unit may spend a standby token to perform a free move action or a free attack action. The only unit that can be declared as a defender during a free attack action made in this way is the enemy unit that performed the attack, move, or action that triggered the standby.If a unit performs a move, an attack, an action, or gains a suppression token, it removes any standby tokens it has.A standby token is spent before any effects that trigger after an attack, attack action, move, or move action. A unit cannot spend a standby token until all of an attacking unit’s attack pools have been resolved.
Melissa’s Battle Droid unit performs the standby action and gains a standby token. Later in the round, the enemy Clone Trooper unit activates and performs an action. Because the Clone Trooper unit is at of the Battle Droids and in LOS, after the action is complete the Battle Droids may spend their standby token to perform a free move or attack action after the Clone Trooper’s action is resolved.