Waterfall project is a temporary effort undertaken to create a unique product, service or result. A lot of  changes are performed in projects You assign one cross-functional project group to steer and another to perform the project-
The Waterfall model is an ideal model so when you run a project acording to it it will never work without exeptions and diviations. Running a project is activly deling with risks (the risk that a solution do not work as entended). So the chalings for the project leader is to manage the risks and keep them under controll. The Setbased technique described i Building blocks is a way to mitigate risks of failiers learning to know before desiding.to perform the project