If you take the Pavlova out the oven too soon while it's still warm this will cause it to sink, collapse and shrink from the cold air. To help the oven cool down quicker, we open it just a crack so the hot air can escape faster without allowing the cold air to come in too quickly. If you're worried, leave the pavlova in the oven overnight with the door shut then top it first thing in the morning. Don't worry though, pav is such a forgiving recipe - even if the meringue cracks or sinks in the middle, once it's covered in whipped cream and toppings no one will ever know. And no matter what it ends up looking like, it always tastes amazing!