Quote Originally Posted by David Brown
Absolutely! There is a device that "unlocks" the lens. When the lens is unlocked, a baffle slips into place to block light from getting to the film with the lens off. You can check this by taking a lens off and looking inside the camera body. Turn the lock on and off and the baffle should move into and out of the way.

Clear as mud? Hope that helps. Maybe someone else can explain it better.

Cheers

David
No, you explained it perfectly - thank you!