[SargeNZ]

From just thinking it through you could conclude that spades help with both up and down deflection. I would assume that with the aileron neutral you want the spade with a 0 degree angle to the oncoming airflow? Then if the aileron deflects down, the spade is put in a positive angle to the airflow, making lift, since the spade is foward of the hingepoint, it helps the aileron rotate down.

When the aileron is deflected upwards, the spade is put into a negative angle, this causes a force downwards on the spade, helping to pull the aileron upwards.

Would it not also add to aileron surface area and thus roll authority?

NB: This is just what I have deduced, and could quite easily be wrong.