What is Flexible defense?

The flexible defense is a military theory about the design of modern fortifications. The examples of "flexible" defense-lines are not based on dense lines of heavily armed, large and expensive concrete fortifications as the systems such as the Maginot Line were. Their protective capacity hinges on multiple lines of obstacles and small shelters fitting into the environment. They are "flexible" because soldiers are not locked in pillboxes, but fight instead in easily replaceable open earth-wood made positions, while bunkers serve only as shelters during bombardments. As a result, they are able to adapt to the opponent's movements, and there are no easily targeted large buildings in these lines.