April through June in Tuni is not gentle. Classes that ran outdoors in January move indoors, water breaks get longer, and Guruji's morning sessions shift earlier and earlier to stay ahead of the heat. It is also, without fail, the season in which Sheetali and Sheetkari get taught to every single new student in the room, whether or not it was on that day's plan.
The reasoning is not folklore dressed up as physiology. It is closer to the reverse: a genuine piece of thermoregulation that the tradition happened to notice and name centuries before anyone could explain why it worked.
What is actually happening in the mouth
Roll the tongue into a tube, or — for the majority of people who cannot roll their tongue — draw air in through gently clenched teeth instead, and the incoming breath crosses a wet surface before it reaches the lungs. Evaporation there pulls heat directly out of the blood moving through the tongue and the vessels just beneath the palate, blood that is on its way toward the brain. It is the same principle that cools a wet cloth laid across the forehead, run through the body's own plumbing instead of a piece of fabric.
The effect is not subtle to anyone who has tried it honestly. Nine to fifteen slow rounds, practised in the shade after the worst of the midday sun, produces a drop in felt body temperature that most students notice within the first few rounds — long before any effect on core temperature could plausibly register on a thermometer, which tells you something about how much of heat distress is nervous-system reactivity rather than degrees on a scale.
A caution worth repeating
The same practice that makes June bearable is the wrong thing entirely by December — this is not a technique to keep in year-round rotation. Guruji is direct about this every season: Sheetali and Sheetkari come off the schedule the moment the evenings start to cool, and Surya Bhedana or Bhastrika, which do the opposite job, take their place. The tradition was never offering a single best practice. It was offering a full set, and trusting the practitioner to notice which season they are actually standing in.