How does a Poolish work?
A Poolish combines equal parts flour and water (by weight) with some yeast (the amount varies according to the expected length of fermentation time, using less for longer, slower fermentations).

The fermentation is then carried out at room temperature for periods of time long enough to bubble up and increase its volume. At its peak, the volume starts to fall back slightly (recede) with the appearance of a wrinkled surface. This process can take anywhere from 3 to 15 hours, depending on the inoculation level.