[Mr70]

I think because this system is designed to provide vacuum only in a transmissions final gear,the intake mounted solenoid is "energized",(Turns On),right away when you turn the key to ignition.This vents the solenoid to the atmosphere and keeps any vacuum from reaching the distributors advance.
Once you hit the desired gear,2nd,3rd,4th or reverse,depending on what Trans.type you have,the ground field collapses,(Turns Off),via a trans. mounted switch and allows vacuum to now reach the distributors advance.
There's also a firewall mounted relay & a hot/cold temperature switch mounted in the head,that will override and collapse the field,(Turns Off),and allow immediate vacuum to the advance in all gears in the event the temperature goes above/below preset degrees.