The aperture is listed on screen to see the process in action. If you would like to read more about the technical details look back at these posts: Eliminate Flicker and GBTimelapse/eMotimo Aperture Ramping.
Stick around after the video to see an explanation of the aperture pull/ramp.
The figure above shows how the aperture was smoothly ramped from f/22 when the sun was 4° above the horizon to f/2.8 when the sun was 5° below the horizon during twilight. You can specify the start and stop points as well as the curve to follow, (linear, logarithmic, or cosine as shown here). I like to use the cosine curve because the gradual change at low f-numbers makes lens vignetting changes less noticeable.