The idea is a build a basic hull using cheap components that can be built quickly. Then, use one or more retrofits to bring the hull up to the finished design. Each retrofit design must cost no more than 150% of the total cost (cost of all 3 resources combined) of the hull from its current design. For this reason, a series of intermediate designs is usually required between the orignial and finished design.
The costs of the retrofitting process incur a significant penalty over that of simply adding the final design to the build queue, but the spaceyard that built the hull is quickly freed up to turn out other hulls while the new ship's components are repaired.
Some players regard this technique as somewhat gamey or exploitative, but many others regard it as valid because there is a trade-off involved.