The system that I recall involved a barostatic unit that released the drogues from stabilising the seat and allowed them to pull directly on the parachute to haul the crew member from the seat for a normal descent. The barostatic unit triggered at 10,000 feet (oxygen level) and ran down over two seconds then operated a scissor link to release the drogue from the seat. They had a powerful spring operated bolt and while testing a batch I once advertently omitted the bolt retainer on the test fixture. The bolt fired straight across the workshop and right through the frosted glass wall into the passageway. Thus began my acquaintance with the RAF's last flying Lancaster.
I believe that later baro-release mechanisms use compressed gas to release the scissor link.