I recently had to move a film roll from one back to other mid-roll (6 shots done) on a Hasselblad A12 back. I took the old back inside a dark bag and rolled back the film to the unexposed spool. Then, I loaded the film in the new back and fired the shutter with the lens closed till I reached the last known good frame. I however saw that the indicator on the new back did not move until I reached the "new" frame and just proceeded to the point where it looked like just 1 frame was shot. I always thought the film indicator depends on how many cranks had happened on the back, but it looks like that is not the case.
Can anyone explain how the internals work for the film indicator? I am now incredibly interested in this Swedish engineering.