Every amplifier is advertised with power at one load — “1 W into 32Ω” — and that number tells you almost nothing about what it does with your headphones. This works out the rest of the curve, and shows you where the amplifier stops being limited by voltage and starts being limited by current.
An amplifier has two ceilings. It can only swing so many volts, and it can only push so many milliamps. Which one you hit first depends entirely on the headphone you plug in.
Into a high impedance load the current stays small, so the voltage ceiling decides everything — and power falls as impedance rises. Into a low impedance load the voltage needed is tiny but the current is not, so the current ceiling takes over — and power stops rising, or even falls. The impedance where the two ceilings meet is the knee, and it is the single most useful number about an amplifier that nobody prints on the box.
This is why a dongle advertised at 250 mW into 32Ω can be embarrassed by 300Ω headphones, and why an amplifier that looks weak on paper can drive them comfortably. The advertised figure is measured wherever it looks best.
Once you know the power available, the power calculator tells you how much your headphones need, and the matcher does both at once across every source in the database.