They simply don't. That trick never works.
Williamson v. Sumner « Uneasy Money: Williamson makes another curious argument based on a comparison of Hodrick-Prescott-filtered NGDP and RGDP data from 1947 to 2011. Williamson plotted the two series on the accompanying graph. Observing that while NGDP was less variable than RDGP in the 1970s, the two series tracked each other closely in the Great-Moderation period (1983-2007), Williamson suggests that, inasmuch as the 1970s are now considered to have been a period of bad monetary policy, low variability of NGDP does not seem to matter that much.
Marcus Nunes, I think properly, concludes that Williamson’s graph is wrong, because Williamson ignores the fact that there was a rising trend of NGDP during the 1970s, while during the Great Moderation, NGDP was stationary. Marcus corrects Williamson’s error with two graphs of his own (which I attach), showing that the shift to NGDP targeting was associated with diminished volatility in RGDP during the Great Moderation.
Furthermore, Scott Sumner questions whether the application of the Hodrick-Prescott filter to the entire 1947-2011 period was appropriate, given the collapse of NGDP after 2008, thereby distorting estimates of the trend…