The Italians would call the opening section the Passagio which was also a common feature in the North German Praeludia. However, it is questionable whether the Italian term is approprate in the French style composition. Basically it is a virtuosic episode written in a monophonic texture where we can find both the elements of arpeggio and scale-based passages. At any rate, even at this early stage of Bach's career, the composer shows a unique angle of blending multi-cultural elements in one work.
In the longest main central section we can hear very imposing stepwise rising theme in long note values which is treated in a fugal manner in various voices. This is a typical French 5 part texture, because the French employed 5 stringed instruments in an ensemble (2 violins, 2 violas, and a violon). Therefore, many of the French classical type of compositions are written in this texture as well (especially the fugues). Can you guess what kind of ominous chord sounds at the end of this section?
Pièce d'Orgue ends with a virtuosic but a little slower and heavier texture which have 5 voices encoded: 4 voices could be percieved in both hands and pompastic and magnificent Dominant pedal point in the pedal line.