It is clear that the piece is actually in F minor, but only this third movement has three flats written in the key signature (F minor originally has four flats), which is a mystery.

F minor is the key that expresses the most sorrow among the keys Bach used. This can be understood by recalling other F minor works by Bach, such as the Inventions, Sinfonias, and The Well-Tempered Clavier. In just intonation, four flats were the absolute borderline for maintaining a beautiful sound; it was not possible to add more. Therefore, F minor was a special key that required a certain readiness from the listener.

Furthermore, in this era, chromatic progression was a kind of taboo and a violation in composition. However, composers sought to express a world of sorrow and despair even by using it. Chromatic progression was truly a distorted and rare world. In this piece, chromatic progressions are written in abundance, to an almost overwhelming extent, clearly depicting human wailing and an uncontainable sorrow.

Therefore, performers should play this movement as heavily as possible, expressing a poignant cry.