It makes quite a soft, pleasant note if you blow gently or a harsh high-pitched note if you blow harder. I believe this phenomenon is called 'overblowing' - it's effectively producing the second harmonic instead of its fundamental frequency. Not all of the whistles I've made do this. I think it has something to do with the ratio of the length of the resonating chamber to its diameter.
I was going to explain how I made the Barley Twist pattern but I've just realised I can't remember exactly! It involved wrapping pieces of masking tape around the whistle in a spiral pattern and gouging along the edges of the tape. I think it has four 'starts.'