pounds of boost was a guess
a wise man once said:
Basically, at 0, it means that the pressure in the lower intake is even, meaning that the SC is feeding air at the same rate that the engine is pulling it.
In a naturally aspirated engine, there is always vacuum, as the engine pulls the air in rather than being fed. Basically, it would always read negative pressure (vacuum).
In the SC'd car, it is unecessary at idle/low rpm'* for the engine to be constantly fed air. The engine can pull it fine.