Thomas, thanks for posting this!
I'd never heard of the diving reflex before. It sounds like it might solve the "brain-scramble" that I've posted about elsewhere
One night when "brain-scramble" got especially bad, I tried putting my face against a ziploc bag filled with ice cubes for one minute. I had forgotten the part about holding my breath, but you basically have no choice. It did not induce the instant calm that a lot of people have reported elsewhere, but it may have induced some calm during the following hour.
The next time I experience strong brain-scramble, I'm going to try it again, but with a big bowl of ice water. A couple papers, such as "Mechanism of the Human Diving Response"
(Gooden, 1994), report that the effect is much reduced when you use a plastic bag. Here's a video
that shows some students directly immersing their faces in cold water and monitoring as their heart rates fall from about 120 bpm to 60 bpm. They report that the water felt comfortable and they did not have to struggle to hold their breath (unlike my experience with the ziploc bag).