What is falling Shepard's tone?

A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower.

Click to read more on it. Keeping this in consideration, how does Shepard's tone work?

The Shepard tone seems to continually rise or fall in pitch. The sound of the Shepard tone is, in reality, a cycling between a limited set of tones, each separated by an octave. The illusion works because each tone seems to sounds lower than the preceding.

Secondly, what are fractal tones? Fractals are tones with harmonic and semi-predictable relationships generated by a recursive process. The tones are pleasant, follow the “rules” governing relaxing music,¹⁰ but are not associated with tunes that the listener may hold in memory.

Regarding this, what is a Shepard tone and why is it strange?

The Shepard Tone is an interesting audio illusion that creates the impression of an always rising or falling pitch that really doesn't get anywhere. Despite feeling like always going up or down, is always stuck in an eternal auditory fractal.

How does an auditory illusion work?

In auditory illusions, the human brain thinks that it can hear something that is either not “there” or exists in a very different form to how it is perceived. They could just be momentary disconnections in the way our minds and ears interact, as the brain processes fragments of sound into a continuous whole.