Canonic Variations on "Vom Himmel hoch da komm' ich her"
This is a two part canon between the right and left hand. It is a “canon at the octave” because the second part starts an octave lower than the first part.  The hands play on different manuals so that they can produce a different sort of sound, like a duet between two instruments.  The chorale tune (cantus firmus) is heard in the pedals.  It is played in long notes with gaps of a bar and a half between each phrase.  The music of the canon, played by the hands, sounds in parts like the shape of the chorale.  It starts with six notes going down.  Bach may have done this deliberately to suggest Christ coming down to earth.  This sort of “symbolism” or “word-painting”, in which the actual notes of the music show the meaning of the words (or what were the words in the original hymn), is very common in Bach’s music and in a lot of other music of that time.