Christensen compares the education system with a production model. He introduces two production models: interdependent and modular. Interdependent models are not robust in the face of change, such as an assembly line. If one part is imperfect the whole thing falls apart and needs to be redesigned. Schools as they currently exist are interdependent. Time, hierarchy, environmental and material constraints, and lateral interdependencies (where in order to change the way one subject is taught you must change the way another is taught). If students don't fit the mold of this factory model either their education has to be redesigned (as is the case with special ed. students) or they get left in the dust. Customizing education for each student in this model is so costly from both a monetary and resource standpoint it's impossible to achieve.
The second model is much more robust. In a modular model, the design is such that it can be easily customized. Christensen gives several examples of modular design. One is a light bulb. Almost all lamps have the exact same light bulb interface. As long as a light bulb can be screwed into the socket, it can be any shape or type. You could use an incandescent bulb or a CFL or even a blacklight bulb and it fits the same lamp. The standard interface allows for easy customization of your lamp.
At the end of the chapter Christensen introduces the idea that technology may be the way to convert schools to a modular model. I have some ideas on how that might be but I am curious to find out what he has to say.