A common and effective project management technic is to assign more people to the team if the project gets behind schedule?

Yes and no. When projects come to the crunch point it is very tempting to throw resources at the problem. However the reality is in many instances more resources are not available with the right type of experience, or else will make no difference. After all say in software development more developers doesn't equate to faster development or better quality. Please see the links below for more information.