If a unit that has missile weapons is charged then it can respond by shooting at chargers coming from more than half their charge move away. This is called 'stand and shoot'. These shots are worked out once all charges have been declared and before the chargers are moved. Work out the shots at the measured range and apply the -1 to hit modifier for shooting at a charging target.
A unit which stands and shoots does so out of the normal turn sequence, in the other side's movement phase before chargers are moved. Note that chargers who begin their move within half their charge distance of the enemy cannot be shot at in this way. They are too close, and there is insufficient time for the the unit to shoot at their enemy.