the trick is explained at realclimate.org:
No doubt, instances of cherry-picked and poorly-worded “gotcha”
phrases will be pulled out of context. One example is worth
mentioning quickly. Phil Jones in discussing the presentation of
temperature reconstructions stated that “I’ve just completed Mike’s
Nature trick of adding in the real temps to each series for the last
20 years (ie from 1981 onwards) and from 1961 for Keith’s to hide
the decline.” The paper in question is the Mann, Bradley and Hughes
(1998) Nature paper on the original multiproxy temperature
reconstruction, and the ‘trick’ is just to plot the instrumental
records along with reconstruction so that the context of the recent
warming is clear. Scientists often use the term “trick” to refer to
a “a good way to deal with a problem”, rather than something that is
“secret”, and so there is nothing problematic in this at all. As for
the ‘decline’, it is well known that Keith Briffa’s maximum latewood
tree ring density proxy diverges from the temperature records after
1960 (this is more commonly known as the “divergence problem”–see
e.g. the recent discussion in this paper
<http://www.realclimate.org/index.php/archives/2008/09/progress-in-millennial-reconstructions/>)
and has been discussed in the literature since Briffa et al in
/Nature/ in 1998 (Nature, 391, 678-682). Those authors have always
recommend not using the post 1960 part of their reconstruction, and
so while ‘hiding’ is probably a poor choice of words (since it is
‘hidden’ in plain sight), not using the data in the plot is
completely appropriate, as is further research to understand why
this happens.
http://www.realclimate.org/index.php/archives/2009/11/the-cru-hack
Les