Contrast is not two sensations. It is one system, opening and closing on command.
In the heat, the vessels near the skin widen. Blood moves outward, toward the surface, to shed warmth. In the cold, they narrow, fast, pulling blood back toward the core to keep it. Move between the two and you are asking that system to work — to open, to close, to open again.
This is the mechanism under the ritual. Not magic, not detox, nothing that needs a better word than what it is: a pump, exercised. The widening and the narrowing are the repetitions. The threshold between is the rest.
You feel it as a flush, then a bracing, then a strange, level calm. What you are feeling is a system that has been asked to do its oldest job, and has.
A reading from Meridian.