Purpose:

1) To make the capillaries better able to move blood around the body, reducing the amount of work the heart needs to do. This also increases the effectiveness of Wim Hof and other hyper-oxygenating breath practices.

2) To increase one's mental toughness and focus. To practice taking action even when one doesn't want to.

Practice:

This one's pretty simple. After a hot shower, I position my neck and back directly under the water. Then I start making the water cold a little bit at a time. I don't ever go ice-cold. Just past lukewarm to where the cold is noticeable but not overwhelming.

I've found that doing the cold exposure before the shower is mentally easier, since I know I'm about to turn the hot water on. However, the cold exposure is physically easier after the hot shower for some reason I don't understand yet.