Because Rule 240 is interpreted differently by each airline that has one, it is best to check the policy of the airline that you'll be traveling on.
Arm yourself with a copy of the policy and have it with you at the airport. Some airlines may need to be reminded of their responsibilities. Others will be more fastidious and take care of their passengers, valuing their passengers' business.
When an airline is responsible for the delay or cancellation of your flight it is important to be aware of your rights as a passenger. So tote a copy of the Rule 240 for the appropriate airline (or a detailed contract of carriage for those without Rule 240 provisions) and hopefully you won't ever have to use it.