The Stacker card modifies the function of the five-way pickup switch such that position three activates the bridge and middle pickups in series rather than the middle pickup by itself. This gives you a sound similar to pickup switch position two (bridge and middle) but with more output, gain and low end. Also, four internal DIP switches let you select two different "treble-bleed" circuit modifications (both reduce treble loss when lowering volume) and two different tone modifications that increase the amount of treble cut affected by the tone control knobs.
Position 1 - Bridge
Position 2 - Bridge & Middle
Position 3 - Bridge & Middle in Series
Position 4 - Middle & Neck
Position 5 - Neck