This is one of the great pieces Bach wrote for organ, played on an organ from Bach's time. The numbering system of Bach's music runs from zero to a thousand or so, and it is convenient for those like me who get confused between one A minor Prelude and Fugue and the other.
This piece is in A minor. It is an impish, even demonic work. Modern fundamentalists would be right to be scared of its power, something the faithful in Bach's time savored rather than feared.
Bach's music is really rhetoric. It makes an argument, an argument more eloquent than any possible in English. The closing chords are the final, irrefutable statements of the arguement. The performer, John Scott Whitely understands that, and finally shows a little emotion at the end.
Like most of Bach's pieces, BWV 543 is best understood if listened to many times by many interpreters. This recording is a good start, however.