Upon further investigation I've found that two electrical components interlock the shift lever. A shift lock soleniod and a shift lock relay. The solenoid is energized and locks the shift lever when the key switch is in the on position. When the brake pedal is pressed the shift lock releay is energize and the rely contact opens the shift lock solenoid circuit (de-energizing the solenoid) releasing the shift lever to move from park.

I suspect, but haven't confirmed that the key switch is mechanically interlocked with the shift lever button.

When you select the "ACC" position the shift lock solenoid isn't energized and the mechanical interlock is released allowing the shift lever to be moved to the neutral position. Then the key switch can engage the starter and your in buisness.