This rule appears to confuse many artists or is ignored entirely 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 about the last applied paint film being more flexible than the paint film underneath. Another way to clarify this rule is to add 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 any more 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 begins to lose weight, the rate of loss slowly diminishing." An increase or decrease in the 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 more significant than the top layer, the top layer will inevitably become disfigured as a result.
Learn how to apply the fat-over-lean rule in your painting by visiting the Painting Best Practices
Read more about this rule and a better explanation in Confusing Concepts In Oil Painting: Fat Over Lean.
Read the book by A.P. Laurie, The Painter's Methods and Materials.