it works because there is effectively ‘float’ associated with each gear shift. The hub allows the cable tension to vary by around 2mm either side of the ideal position.
The shifter cable pull does not exactly match that required by the hub, but it is within afew mm. Therefore, by making the double shift we can move our shift position from the effective “low side” of the 1st gear range to the “high side” of the second gear range. With the next single shift taking us into the 3rd gear range. If we break the gear ranges into 3 discrete positions for each gear its easier to visualise.
Starting at position one, the double shift takes us to position 5 or 6 (2nd gear). Then, the next single shift takes us to position 7 or 8.