Dare I plug my website?
www.ejectorseats.co.uk 
It all depends on what seat you are being seperated from really. The latest seats (Mk 10 to Mk 16) use cartridges to generate gas which does things to pistons/cams/detents etc and releases the retraction straps and lap lugs etc thus allowing seat to fall away.
Older seats simply undid the seat harness and the parachute was attached to a seperate harness worn under seat harness. This changed with later Mk 3 seats onwards.
Pulling the manual seperation handle bypasses the system and fires the same cartridges to have the same effect if you need to get out of seat earlier that the system says.
Non martin Baker seats have similar systems but same effect. The Escapac seat (USA) uses a rocket to seperate pilot from seat!!!
