I probably need the Complete Idiot's Guide, but what I get out of this is, using the mean of the whole data set (if it does have an actual
hocky stick shape) as zero creates a higher horizontal line from which all the data vary in various amounts & it tends to «pull up» the negative differences & makes the positive differences look not so big (or it makes all the data look on average equally large in distance from the mean, both in pos & neg directions), making the whole thing look like nothing much is happening, aside from cyclical changes.