Do you whistle to your horse as a verbal aid?

My trainer has always done this with Mag. She whistles a tone that lowers to request him to slow down. I thought it was just her but I just watched a video of a German trainer teaching a horse to trail ride and he did the exact same whistle. (Here is the link to the video, at :53 he whistles and brr's the horse. He explains he's letting the horse go for demonstration purporses, to show that this horse will just go faster and faster to get away from stress.) 

I asked AI and it said it's not just Germany. Link explaining it. 

When I moved here I noticed there is no German word for "whoa/ho." They blow through their lips and make a brrr sound. It sounds silly to me but the horses know what it means. 

Now I guess that whistle is also a cue. 

Have you ever done this in the US or Canada, or heard anyone do it? Curious!

Here's a pic of me today enjoying the sunlight shining on my yarn, warming it up. Such a pleasant sensation while crocheting!

And here's Bellis today doing what she does on every sunny day in Winter - lining herself up to the sunshine sideways to get every bit of warmth into her body.