Thanks for the replies
Thanks - Here is a picture showing the button.
When I press the button and turn the wheel (CCW) until the upper shaft locks (I had the top off the machine to see that the button shaft was going into the slot in the main upper shaft) once lock the wheel will only turn (CW) about one full turn. If I release the button and then stitch it doesn't seem to change the stitch length. If I then press the button and relock the shaft the wheel will only turn (CCW) - back to where I started. Stitching again there isn't any change in stitch length. So can I properly assume that this is resetting the clutch - I don't think I've had the experience of engaging this clutch (which I thing is a good thing not to have had!).
I'm new to this machine and learning how to set things up. I've had some tension challenges when I change materials and threads.
I have the 30-444G Service manual however I thought that if there was a users manual it may provide descriptions on how to operate the button in question.