I look at it like how a boxer reprograms themselves not to flinch when a punch is thrown at their face through repeated conditioning. Instead of instantly regretting the cold, you learn to accept it and not pre-program your reactions to it. Since most people live in 70 degree temps all the time, walking out into sub-freezing weather with minimal clothes is an instant shock to them.
I am just some weirdo that walks around in the cold barefooted and in shorts all year around, and no expert in reprogramming their parasympathetic nervous system or commanding my white blood cells to attack things. The WHM method just allows me to do it longer and in colder weather. YMMV and all that.
In is full, out is normal.
Disclaimer: I've trained 4 days with Wim for one of his landmark experiments. I was one of the guinea pigs.
The only thing that still annoys me is rain and hail at times. But as a chilly/shivery person the issue was always cold temperatures for me. That issue is gone. I say it with surprise because I still think it sounds crazy. Before 2014 it had such an impact on my life and suddenly all that misery just drops to zero.
Spiritually though, I didn't notice a lot, I'd rather stick to meditation for that. And whatever people may say, IMO, the Wim Hof Method is not a meditation. It's a combination of two breathing techniques + an optimistic trained/taught mentality about the cold being your friend.
Oh, I also sometimes use it as a natural painkiller when I don't have one on me and start to get a headhache. You know since the adrenaline-like state includes some actual adrenaline [1] :)
One "issue" with the WHM is that you need to keep doing it to stay in the state. It otherwise wains after 5 minutes or so. Because of this, it's impossible for me to be social in a normal sense, since breathing in a very specific way + talking at the same time is about as tough as beatboxing while singing (which I know how to do as well :P). Which means: it requires a lot of practice and trickery.
[1] Research shows you get a lot of adrenaline in. It's in the publication I was a guinea pig for: https://www.pnas.org/content/pnas/111/20/7379.full.pdf
I'm more likely to go swimming, even when the water is cold.
I'm more likely to take a cold shower, which has many benefits.
Just generally being uncomfortably cold less often, as others have mentioned.
And that general good feeling of "I didn't think I could do that, but I did!"
I similarly remember willfully altering my heart beat at the doctor's office on several occasions as a practical joke. I wonder if the two situations are linked in a significant way?
I do wonder how effective I actually was, as it isn't really something I have thought about for literal years.
Sounds like some serious hyperventilation - I wouldn't recommend doing that as it can lead to cerebral hypoxia
Deep breathing. Not slow, but full and continuous inhaling and exhaling to the boundaries of lung capacity.
Physically and mentally relaxing. Shivering is a way to generate heat through motion. To counter it, and stimulate chemical heat generation, relax all the muscles. To relax mentally, something like breath meditation.
Mental/visual. Imagine being warm, like on a hot beach laying on the sand.
Hope this anecdata helps someone. I wouldn't count on it replacing warm clothes, but it's nice in a pinch.