Graphic © 1996, Timothy A. Smith, used by permission.
Here the upside-down second signature in the lower staff indicates that the second voice is to be played upside-down. Voice 1 starts on C, and Voice 2 starts on G (a perfect fourth lower) at the sign and moves in the opposite direction from the first. Again, a modified Royal Theme plays in the top staff, in harmony with the two lower voices. Note that here Bach scored the canon in 4 measures and had the second voice start halfway through the first measure.
To make a function run upside-down only requires a minus sign. In this
example the blue function g is defined from the red function f