Twist the whole cluster off at the base before the cap edges flatten and curl; a flat cap is shedding spores and tears easily. The DMR folder adds a health rule: open doors and windows two hours before harvesting and wear a mask, because oyster spores cause allergy.
A flush is one wave of mushrooms. The folder says the first flush gives about half or more of the total, the second comes 8-10 days later, and three flushes are worth keeping; then the bags go to a deep pit.