Are you talking about a slider switch that's underneath the lens? If so, that controls the shutter giving a T (for timed) option it enables the shutter to be fixed open for long exposures, and nothing to do with f-stop apertures.
If you are talking about something different, then I have no idea. It's not something that's available on my PS lenses, and isn't mentioned in my Sqai manual.
Confused of Canterbury.