My understanding of Counterspelling is that it adds a pool of dice to your defence. That means if you try to hit someone with an indirect combat spell she makes a defense test with reaction + intuition + counterspelling assuming Spell defence was declared. The mage has a counterspelling pool equal to her counterspelling ranks and can spend as many dice of this pool to defend as she wishes. Furthermore my understanding of counterspelling direct Spells is that, since there is no defense test, they get added to the damage soak test. I'm not sure about this one since I can't find any rules for counterspelling and direct Spells but since the example on page 295 mentions that it is possible I assume that they get added to the soak test.
Now with an indirect area spell I'm not sure. According to page 283 they work like grenades so the caster has to make a Spellcasting + Magic [Force] (3) Test. So there is no defence test since just like grenades you can't dodge them unless with certain interupt actions from Run & Gun. Would counterspelling be rolled against the attack and decrease the amount of net hits and might cause scatter or would it be added to the soak similar to direct spells?