As Jetdriver mentioned, pressurization schedules (differential pressure, climb/descent rates)are programmed into auto controllers with manual backups. As the aircraft descends, the cabin will depressurize as a function of the amount of altitude (aircraft and cabin) to be lost prior to landing. Most jet aircraft land either depressurized or extremely low differential pressures (which is relieved on touchdown). Couldnt open the plug doors otherwise.
One little bit of pressurization trivia. Some modern (corporate) jets are operating at differential pressures over 10.0 psid. Airliners run around 8-9 psid. The folks who design nuclear weapons consider the "ring of destruction" around a blast the .5 psid overpressure ring. The forces involved in a pressurized cabin are enormous - that is why very large things can be forced through very small holes when there is a failure.
Shore Guy