Pretty intimidating (at first)
My favorite story is the one when I first got my whistle from Border Collies in Action. I worked with that whistle for several days sputtering and spitting to make a noise. Nothing. I got on the phone and called Border Collies in Action and told them I thought I might have purchased a "defective" whistle. Imagine the HUGE grin on the person on the other end of the phone! Imagine my red face when she told me to continue working on it. hahahaha
Easier said than done.
What I finally figured out was "blow what you can blow." I had to go with whistles on my first dog that were ones I could blow. Never mind that I am completely tone deaf. On my second dog I changed up a few whistles. I'm working my 3rd dog and I have kept what I used on my 2nd dog. I'm not completely happy with them, but it's the best I can do.
Your dog understands the whistles even if they come out full of spit and don't sound quite right. They get used to "bad" whistles. They know what you are trying to blow, so don't get too stressed about it.
And, for those of you with those clear, melodic, finger whistles...I applaud you. It ain't gonna happen here. Not in this lifetime.