We vary the time between hops, the "dwell", at any one station. This varies the end-to-end message travel time and thus the number of in-flight messages and number of dots on the map.
We also vary the d3 "transition" duration to make the display more or less jumpy. This is purely an artifact of the display and has no effect on other numbers.
Note: Dwell is in model time viewed at 10x in playback.