Indian recipes routinely give cooking times as a number of whistles rather than minutes. That convention exists because the whistle marks something more reliable than the clock does.
What the whistle actually is
A weighted valve sits on the lid and stays seated until steam pressure underneath is strong enough to lift it. When it lifts, steam escapes with a sharp sound and the weight drops back.
Each whistle therefore records the same physical event: the vessel reaching its design pressure and venting the excess. It is a pressure signal, not a timer.
Between whistles the contents sit at that pressure, and therefore at a temperature above the normal boiling point of water. Cooking rate depends on that temperature far more than on elapsed minutes.
Counting events beats counting minutes
A clock started when the cooker goes on the burner includes an unknown warm-up period that varies with burner power, the amount of food and the starting temperature of the water.
Counting whistles skips all of that. The first whistle confirms the cooker is at pressure, so subsequent whistles measure time spent actually cooking rather than time spent getting there.
The interval is not fixed
The gap between whistles depends on how much heat is going in. A high flame vents frequently; a low flame under the same cooker vents rarely while holding the same pressure.
This is why the instruction is usually a flame level plus a whistle count. Three whistles on high and three on low are not the same amount of cooking.
It is also why whistle counts translate poorly between kitchens. A recipe written for one cooker on one stove is an approximation on different equipment.
American equipment breaks the convention
Electric multicookers regulate pressure without venting audibly, so there are no whistles to count. Their timers only begin once pressure is reached, which restores the same logic in a different form.
US stovetop pressure cookers often use spring valves that release continuously rather than in bursts. They hold pressure steadily but give no countable signal.
Converting a whistle-based recipe means finding the equivalent held time at pressure, usually a few minutes per whistle depending on the flame level specified.
Pressure release is part of the recipe
Letting the cooker cool on its own keeps the contents above boiling for several more minutes, which continues cooking. Releasing pressure manually stops it almost immediately.
Dal that is meant to fall apart relies on natural release; vegetables that should keep their shape do not. Because the appliance holds hot pressurized contents, the manufacturer's own operating instructions are the reference for how to open it.