Venting, also called airing, is the day you stop the mycelium spreading and ask it to make mushrooms. Until then the room is warm, wet and closed: DMR runs the case run at 23 to 25 C air, 95 per cent humidity and about 7500 ppm of carbon dioxide with no fresh air.
At venting you bring the air down to the fruiting band and open fresh air until carbon dioxide falls to 800 to 1000 ppm. The fungus reads the change and begins to knot on the casing surface. Pinheads follow within a few days.