If the POH doesn't tell you, look up the part number for the propeller and grab the manual for that from the manufacturer's website. The C182 POH I just looked at said it uses the McCauley C2A34C204.The manual for that propeller is on McCauley's website. The key information is this:
The C200 and C400 series propellers are constant speed type. They are a single-acting unit in
which hydraulic pressure works against the forces of springs and the natural centrifugal moment
of the rotating blade to provide the correct pitch for engine load. Hydraulic pressure causes the
blades to move toward high pitch (decreasing rpm). The springs and centrifugal moments urge
blades toward low pitch (increasing rpm).
Obviously if you take away the hydraulic pressure, there's no force driving towards high pitch and so the propeller will go to low pitch.