Depends on the system. Fancy ones do that on their own. But older ones are controlled manually: set field elevation and barometer/altimeter setting and the box dumps at a set altitude.

400 series Cessnas I have flown say set the cabin alt to 500 feet above landing field elevation. The Saab was supposed to be at field elevation when landing if set correctly (HAH!). Both dumped with weight on wheels switches if improperly set.