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In a waterfall model, once you have completed a phase, you cannot go back and make changes in it. You only go in one direction without coming back, hence waterfall. You need to finalize Requirements before moving towards design. Once you have moved to design, you cannot change the requirements and so on.
For more info:
http://www.tutorialspoint.com/sdlc/sdlc_waterfall_model.htm
http://www.waterfall-model.com/
