The phrase 'hide the decline' was shorthand for providing a composite representation of long-term temperature changes made up of recent instrumental data and earlier tree-ring based evidence, where it was absolutely necessary to remove the incorrect impression given by the tree rings that temperatures between about 1960 and 1999 (when the email was written) were not rising, as our instrumental data clearly showed they were.
This "divergence" is well known in the tree-ring literature and "trick" did not refer to any intention to deceive - but rather "a convenient way of achieving something", in this case joining the earlier valid part of the tree-ring record with the recent, more reliable instrumental record.
The manipulation of the tree ring data is problematic in two ways. First, it is asserted that the tree ring data corresponds well to global temperatures, and the findings based on the tree rings indicate that there has been a warming of the global climate. However, from 1961 on, the tree ring data indicates a significant cooling of the global climate. As this is apparently contradictory to observed temps, the conclusion must be one the following: either tree ring data are a poor indicator of global temperature, or something else is going on with the climate. Thus one of the three main information sources for the 2007 IPCC report should have been thrown out, or should have caused a general reconsideration of the reports findings.
Instead we have Dr. Jones doing a little 'doctoring' to the record, and the doctoring is hidden in the code, with code instructions such as:
"MXD - but shouldn't usually
; plot past 1960 because these will be artificially adjusted to look closer to
; the real temperatures.
Given the IPCCs statement of purpose:
“The IPCC was established to provide the decision-makers with an objective source of information about human-induced climate change.”
the whole thing is rather exploded.
This is the primary problem with the tree ring data manipulation. The evidence is that a significant effort was made by at least one of the principle proponents of AGW theory to mislead the citizens of the world, and that in and of itself utterly undermines his position.