A linear, time-invariant system - a room, an amplifier, a filter - is completely described by its impulse response[3]: what comes out when an infinitely short click goes in. Measuring it directly with a real click is hopeless (far too little energy), so the practical methods all play a long, energy-rich probe signal and mathematically compress the recording back into an impulse.
Farina's probe[2] is a sine whose frequency rises exponentially with time - the same signal as equation (9) of the generator chapter. Compared to noise-like probes (MLS and relatives) it delivers a substantially better signal-to-noise ratio for the same measurement time (the article[1] demonstrates a ~20 dB advantage in its test setup), and it has one almost magical property: harmonic distortion separates itself from the linear response. Because each frequency occurs at a known time, a distortion harmonic of the momentary tone arrives as if it belonged to a frequency the sweep reaches only later - after deconvolution, all distortion products land at negative delays, cleanly ahead of the linear impulse response, where they can simply be windowed away (or analysed on purpose).
This is precisely the pipeline behind Phonalyser.web's frequency-response module, with one engineering refinement described in the generator chapter: the sweep is pre-rendered, so the deconvolution reference is identical, sample for sample, to what the DAC actually played.