A high-pitched quick sound like that is the opposite of what I use to slow/stop my horse. I'm pretty sure he'd want to speed up if I made that sound. If I want Caleb to slow down I usually use a very low soft hum or talk to him in a deep quiet voice.
Just as an aside.. I only use voice commands on the lunge line. When I'm super talkative one day I might use it undersaddle, but.. meh. It's just another training tool, I really don't see the harm in it =\
A couple of girls at my old barn, who were on the Young Riders team, did it a lot to signal a downward transition. Someone must have taught my horse this too, because he responds very well to it. I only use it when he is excited and jigging a lot, or gets too squirrely at the canter. I will admit to using it when I lunge as he is very responsive to verbal cues on the ground, but I get some weird looks from the girls at my new barn!
I have heard it used by my trainer as well. Every now and again I catch myself doing it too. LOL
Funny how we pick things up.
In fact I had not heard anyone else doing it before and then I watched the vids you posted with Anky. I actually had to put it back and listen again because I was shocked that she did it...
I e-mailed it to my trainer and told her to listen for it. And also because I thought the vids were interesting even tho I do not agree with anything to do with Rolkur and hyperflexion of a horses neck.