The time and circumstances of its composition are unknown. It is transmitted through two manuscript copies. J.T. Krebs, a pupil who produced one of these copies, titled this piece "Fantasie" and subsequently transcribed the Chromatic Fantasia and Fugue (BWV 903). Although formally quite different, a similar atmosphere to the opening section is created by the arpeggiated figuration, where both hands sometimes cross, and the highly chromatic, ornamental melodies.

The work consists of three parts: an opening section characterized by arpeggiated runs, a middle section starting from measure 34, which Krebs labeled "Fugue," and a concluding section from the latter half of measure 92 onwards, which Bach himself likely marked "Presto." Of these, the opening section is divided into two subsections: arpeggios that sweep across the keyboard from end to end like a whirlwind, and a figure consisting of a triad followed by an octave. The middle section merely features short motifs repeatedly appearing in both hands like raindrops, making it too insubstantial to be called a "Fugue." However, the adventurous modulations likely prevent this repetition of the same motif from becoming redundant, enriching the music. Towards the end of the opening section, a B-flat already appears, forming a colorful Neapolitan sixth. In the middle section, it passes through G-sharp minor and C-sharp minor, and in the latter half, B-flat reappears, this time leading to flat keys, namely D minor and G minor. The repetition of motifs is, in fact, an effective technique for highlighting harmonic changes.

This work may have once been considered spurious due to its highly improvisatory nature and lack of motivic development, its unformed structure as a toccata, or its extremely archaic composition. However, precisely for these reasons, it can also be said to be full of a vigor and exhilaration not found in Bach's later, more mature works.