To add to pjc50's comment and to demystify the phenomenon a little...Figure 7 is just a plot of the deviations from a trend line of the GNP. I can't say for sure, but I would bet that this trend line is refit to the data at a regular frequency (probably yearly). Imho, it is useless by itself as a predictive tool....it needs to be combined with other signals to have any real value in signalling a change. It is one of those things that looks like magic in hindsight. To be fair, the article does cite other signals that indicate a downturn and I don't have any insights or desire to call bullshit on the prediction.