Good constant water management is the key word for optimal second flushes.
Water management already starts indirectly when incubated compost is filled and the choice is made whether to use sheeting or not filling depth/weight etc. The water given in the first two days in particular determine how the incubated compost is sprayed to the right moisture level.
At a later stage blow down is very important. With a too high compost temperature of 28-29 degrees Celsius at the start of blow down, too much fresh air or too slowly reducing the compost temperature, too much moisture will be lost from the casing soil. If extra water can be given before and/or in the first flush problems can be avoided. If spraying is done after the first flush, do not take more than two days to raise moisture levels in the casing soil. On the day the first flush is completely picked spray 6 to 10 l/m2 and spray another 2 l/m2 the day after. The casing soil must then be left to dry, so that after 24 hours the shiny surface of the casing soil has changed into a dry slightly dull coloured layer. If the casing soil is sprayed over several days there is an increased risk of bacterial blotch and less fruits develop creating a disappointing second flush.
Jos Hilkens, AdVisie ‘The mushroom cultivation advisors’