The mindset of these people is that they need the requirement to be spelled out 100% clearly and completely. This tends to lock up the project tightly, with difficulty to make changes. It takes too long to define completely what is not clear at the initial stage.
A flexible approach comprises of the following:
a) A clear purpose and a plan to achieve that purpose that is not fully specified. What is vague at the initial stage can be specified at a later date.
b) A willingness to experiment and modify the plan based on experience.
This will develop a lean and entrepreneurial approach.