The Moiré Effect is the result of superimposition of 2 patterns. The Fantaisie Impromptu exhibits a repetitive 4 on 3 pattern, similar in concept to the Line Moiré below where one object is moving relative to the other.
As a result, listeners will frequently feel that the music is moving at a faster pace than what it actually is. It is probably this effect that makes the piece so unique amongst so many others in the musical realm.