In plain English
Same power-law trend, plus a sine that slowly fades (damps) as the years pass — booms and busts that get a bit quieter versus the trend, if that story holds.
If the collector already built the harmonic-gap model, this page shows that. Otherwise it fits a 1460-day sine to the residual and damps it. The lower pane is residual z versus the fitted ẑ.