Joe,
Your Lancaster DOES cock on opening, not on closing like a Purdey. When a fired Wrist-Breaker is opened the front end of the spring/lever is depressed and the rear end rotates the tumbler/hammer back to the cocked position. When the gun is being closed, the cocking dog is raised and since the rear end of the spring/lever is now being held up by the tumbler, which in turn raises up the front of the spring/lever causing the middle portion of the spring to bow down. The cams machined into the action flats are there to increase the bend of the mainspring. In a nutshell, thats how the system works.

Dustin