At the operating side you will see an opening with right to it the figures 12, 24, 36. In the opening you can see an indicator, connected to the funnel you had it about in the contetext of scratching. The funnel thus acts as feeler for the windings.
By reading this "meter", due to its imprecision rather a indicator, one gets an idea of the length of film already fed to the spool. Thus if knowing about the numbers of exposures done, one can by this indicator roughly control the lenght to be cut off and be processed. This is an alternative to opening the camera in the dark and cutting off the lenght already exposed. The daylight alternative of course fits the daylight-handling idea behind this Rondinax.
I understand what it is. I was just confused as to what you meant by "use it," since in normal operation that meter is operated automatically by the film passing through the funnel.
I just cut off the film when it's all the way pulled out of the cartridge. The meter just serves as a nice confirmation that nothing has gone wrong with the process. To be sure, this has already resulted in me developing the exposed tail end of the film when I developed a roll of self-loaded bulk film, but I already did that all the time with my inversion tanks and I don't think it noticeably affects the life of my developer.