The discomfort is the mechanism.
Cold. 2 to 10 degrees. 2 to 5 minutes. Triggers norepinephrine. Flushes the vascular system. Burns fat.
Heat. 80 to 100 degrees. 15 to 20 minutes. Triggers heat shock proteins. Strengthens the cardiovascular system. Spikes growth hormone.
Both are controlled stress. The body adapts by becoming harder to destabilise.