On the ground, when the APU inlet door is closed after APU shutdown, hot air can collect in the APU. The hot air can cause a rotor imbalance or bowed rotor. In rare cases, a bowed rotor can cause rotor rub that might lead to a hung start and APU damage.
To prevent a bowed rotor hung start on the ground, the APU door should be kept open for at least 40mins after APU shutdown. This prevents hit air from being trapped in the APU and allows the APU to be started at any time with no risk of a bowed rotor hung start.
If takeoff occurs during the 40min wait, the APU door can be closed at a convenient time after takeoff.