It means when you push the trigger the first blast is 25 psi and the the longer you hold it. It is at 15 psi.. When I turn my compressor on I let it build the pressure up and let it turn off. Then I adjust the pressure to what I want while I hold the trigger down on my A/B.
This way I know I am spraying at the pressure I want.. Usually the first burst of air is done away from the model so it doesn't really matter what that pressure is. ( if you have a double action A/B. It doesn't matter if the brush is over the model).
Hope this helps