What is negative air pressure in mold work?

It means the work zone is kept under slight vacuum with a machine exhausting outside, usually through HEPA filtration. Air leaks inward through the plastic, not outward into the hallway. That is how spores and demolition dust stay in the room being torn apart.

You should feel air tugging at the zipper, not blowing out of it. If a crew is sanding or tearing drywall with the door open and the furnace running, that is not negative air. That is a fan.