This rule appears to confuse so many artists or ignored completely by others. Perhaps a better way to express the rule "always paint fat on lean" is always paint a slower drying paint film over a faster drying film. Think in terms of the last applied paint film being more flexible than the paint film underneath. Another way to clarify this rule is to think of adding a little more oil in the last application of paint than was included in the paint layer just covered, or not to dilute with solvent the last applied layer anymore than the previous one was thinned.
Drying cracks and "alligatoring" is a direct result of breaking this rule. To help us understand why this occurs, A. P. Laurie in The Painters Methods and Materials (1926) describes the process of the drying of linseed oil as follows: "During this process the oil film is not only absorbing oxygen and therefore increasing in weight, but is also losing certain volatile products of the oxidation, thus losing weight. If a thin film of the oil is painted out on glass and weighed from time to time, it will be found to increase in weight in passing from liquid to sticky, and then from sticky to surface dry. It now begins to lose in weight, the rate of loss slowly diminishing." An increase or decrease in weight of the paint film, represents an equivalent change in the dimension of the paint film. If the changes in dimension of the under layer are considerably greater than the top layer, it is inevitable that the top layer will become disfigured as a result.
Learn how to apply the fat-over-lean rule in your painting by attending the Painting Best Practices workshop.