Climategate: What's the "trick" and what did it "hide"?

Climategate: What's the "trick" and what did it "hide"?
"In a new famous email, Phil Jones, then head of the Climactic Research Unit at the University of East Anglia, wrote on Nov. 16 to climate research colleagues in the United States and at his own CRU:
'Dear Ray, Mike and Malcolm,
Once Tim's got a diagram here we'll send that either later today or first thing tomorrow.
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 (sic) from 1961 for Keith's to hide the decline...'
What's the trick and what did it hide?"