It illuminates many puzzling phenomena that one observes on college and university campuses, and should also prove useful to people planning their careers or trying to understand the careers they have had. The vertical axis has to do with the importance that your colleagues attach to your potential and actual work beyond the Ph.d. The horizontal axis is time. The top left part of the U represents the high esteem that your colleagues inside and outside the university have for you at the time you are hired. And the high point on the right corresponds to the the golden years I just mentioned, which seems to begin about five years after one becomes a full professor.
Several factors conspire to produce the trough.
When you are just out of graduate school, you haven't done much, but your teachers have written inflated letters for you. You are overly-praised pure potential. People find very little to complain about in the potential works that you haven't written yet. If you are adept in your interviews, each of your future colleagues believes that in your works his or her own wonderful ideas will be adequately appreciated for the first time. Of course, once you get a job, you have to start writing and publishing and are judged on the basis of this work.