As I understand it, the key is something called succinate. Its release is triggered by shivering - and it promotes the activation of thermogenesis in brown (high mitochondrial content) fat cells. It might (insufficient in-vivo data) also prompt mitogenesis in fat cells in general, essentially turning some of the white ones brown-er (AKA 'beige' fat).

This would indicate that multiple short bouts of cold exposure (to the point of shivering each time) is likely superior to long periods (e.g. Wim Hoff) of exposure.

I swim in a pool that's kept relatively cool. I shiver a bit at the plunge and in the first lap and use the start of shivering to tell me when I'm done (I suck at counting laps). It's possible that those two bouts of shivering are doing more for fat loss than the energy expenditure from swimming.