The "trick" ("Mike's Nature trick") refers to a technique for plotting reconstructed temperatures (from tree rings) together with recent direct measurements, apparently simply by drawing them in different colours. The code you linked above is a separate technique (labelled "hiding the decline"), a correction for an anomalous decline in tree ring growth, which before 1960 had been closely correlated with temperature. It is speculated that the decline in tree ring growth is due to air pollution or other anthropogenic causes. See here for more detail.