Pop off pressure is when you test for the needle and seat to seal off, holding back fuel when the carb doesn’t need more fuel in the diaphragm chamber because the needle and seat are closed. It’s done with liquid fuel in the needle & seat area. When you run the machine out of fuel, the needle and seat are open, asking for more fuel. No more fuel in the tank, pressure goes through the needle & seat, through the carb and into the intake of the saw.
It happens all the time ....... run your saw out of fuel and open the tank, there’s no pressure anymore