The bonus works like this: You can increase the chances of hitting and/or attack a larger area. Every +1D to your hit chance reduces damage by 1D. Also, every 5' hex you attack reduces the damage by 1D. Single target, normal attack, 8D damage.
Attack a 5' hex, normal attack = 7D damage
Attack a 5' hex, +1D attack = 6D damage
Attack two 5' hexes, +1D attack = 5D damage
etc.
So you can increase odds of hitting and area attacked independently, but any such increase reduces the damage by 1D.
For any targets in the attacked zone, they all roll defence against your single attack.
You can attack non-contiguous areas, but each non-contiguous area counts as a separate action, so are not simultaneous.
Attack 1st 5' hex, +1D to hit = 6D damage and 1D MAP -plus-
Attack 2nd 5' hex, +1D to hit = 6D damage and 1D MAP
Make sense?