Set the Range 2 - (31Hz-850Hz) with VR1 at near minimum, (anti-clockwise)
Set the polarity to negative and the duration to a minimum.
Set the Range jumper to 1 (0.2ms to 11ms) - only the first 1/3 of a turn will operate the servo.
Astable 1 produces a steady stream of pulses around 30Hz
The output of the Astable is connected to the trigger input of the Spike Generator. This is configured to produce a negative pulse on every high-to-low change in voltage on the trigger input
The output of the Spike Generator is connected to the trigger of the Monostable. Every low pulse triggers the Monostable to produce a high pulse which is sent to the servo.
The Spike Generator is needed since the low (space) time of the astable output, is longer than the required. So the Spike Generator shortens the low time to about 200us, which triggers the Monostable to produce a longer pulse. So the "low" time must be shortened before it can trigger the Monostable. Otherwise the Monostable output would stay high for as long as the tirgger is low - too long!