Originally Posted by Craig

Based on my limited understanding; the vacuum modulator only controls the "firmness" of the upshifts and downshifts, not the shift points. The modulator is actually located on the transmission, the valve box on the valve cover partially controls the amount of vacuum suppled to the modulator based on throttle position.

The cable actually controls the shift point as a function of throttle position, making the cable tighter makes it shift later. The electronic kick down switch "forces" a downshift when you floor the throttle at a low enough speed.