Richard Benson explains it very well: The primary sources of manipulation are: 1) Making sure the wrong items are in the index; 2) Taking “hedonics” to ridiculous extremes; 3) Getting consumers to do more of the work and receive less services; and, 4) Changing to a Chain Weighted Index.
If you’re having trouble understanding how the CPI is by no means an accurate measure of inflation, read this. Benson makes it very, very simple.