Ruling Klefki (Ability-Secret Key)

[ale.543]

I was wondering what exactly is Klefki's Secret Key Ability which states "Each of your Fairy Pokemon's Resistance is -40." Does this mean that my Fairy Pokemon would be -40 resistant to all pokemon types, based on the fact that it does not specify resistance type or state that its already existing resistance is the one that is now -40? or does it simply mean that its already stated resistance (usually -20 resistance to dark) would now be -40 resistance to dark? If so, how would this apply if (metaphorically) a Fairy Pokemon would have no resistance or would it just not apply then? And finally, is this Ability stackable?
Here is the card's info,
http://www.pokemon.com/us/pokemon-tcg/pokemon-cards/xy-series/xy3/73/

 

[Mora]

Klefki's Aility only modifies the Resistance's magnitude and not the type. This means that all of the Pokemon effected by Secret Key have their Resistances changed to -40. It does not change the type of that Pokemon's Resistance. So if a Pokemon did not have a Resistance, and Klefki's Ability was in effect, it would not gain a Resistance just because of the Ability. Also, Secret Key reads "each of your Pokemon's Resistance is now -40." It doesn't add to Resistance, it just changed it to a specific number. If it were like Altaria's Fight Song, it would stack because Fight Song is adding to your attack's damage and not setting it to some value.