Four-day workweeks boost days with absent full-time staff by 50% (2 days to 3), creating more demand for overtime hours. That's part of the strategy. That is

Four-day workweeks boost days with absent full-time staff by 50% (2 days to 3), creating more demand for overtime hours. That's part of the strategy. That is magnified by the fact that those workers will preferentially come in mainly/only on the 7th day of their workweek, requiring double paid time rather than just time and a half.