You seem to know a lot about what's inside the smoke detectors here. Except there's no relay, no click, and the test ends as soon as the button is released. And it really does test the sensor.

Some detectors "test buttons" work by inserting a barrier between the emitter (radioactive or otherwise) and sensor, thereby simulating smoke.

The ones here don't do that. Instead, pushing the button activates some circuitry under the sensor, which emits something (particles, smoke?) similar to what the sensor is looking for, causing it to flag an alarm. I cannot see exactly what that is, because it's inside the same assembly as the sensor itself, and I don't want to destroy by opening it up.

However, if I provide airflow across the sensor, then pushing the test button no longer triggers it, indicating that the particles/whatever it generates, are being dispersed before they can trigger the sensor. This "works" whether the air is room temperature, or from a heat gun.

Oh, and the ultimate test: "burnt toast detection" also still works fine.

Cheers