If you've never seen one before, it's a shepherd's whistle. It's specifically for working sheepdogs at a distance.
I have gun dog whistles, and occasionally the spaniels even respond to them when they're not suffering from a bout of selective hearing (commonly known as 'spanielitis'). I also have a walking stick - an 'aide memoir' - for tapping spaniels' bottoms. (Not to hurt them of course, just a little poke to remind them to steady down before the spanielitis sets in). Both are straightforward to operate.
Not so the shepherd's whistle.
I've looked on the internets and found numerous step-by-step guides for operating a shepherd's whistle. But all are about as helpful as Michaelangelo's step-by-step guide for creating great sculpture: you know the "Carving is easy; you just go down to the skin and stop" quote? Mastery of some things require the development of technique based solely on the experience of doing.