Many office buildings had a network of slave clocks which were driven by one master clock.
In normal operation each clock is stepped forward by a DC pulse each minute, but in the last minute any slow clocks are driven by multiple rapid reverse polarity pulses, which align the hands to the 12:00 o'clock position.
This means that the hands of most clocks smoothly step forward, but any incorrect clocks will catch up by suddenly racing to the 12:00 o'clock position once each hour.
In normal operation each clock is stepped forward by a DC pulse each minute, but in the last minute any slow clocks are driven by multiple rapid reverse polarity pulses, which align the hands to the 12:00 o'clock position.
This means that the hands of most clocks smoothly step forward, but any incorrect clocks will catch up by suddenly racing to the 12:00 o'clock position once each hour.