There are several successful approaches to using the digests to locate relevant cases.
Begin with a known case.
From reading secondary sources or using annotated codes, you might have identified a major case on point. Review the topics and key numbers in that case that case; determine which of them (and there may be more than one) address the specific issue in which you are interested. Find digest that covers the jurisdiction(s) you need; the topics and key numbers are the same throughout the system.
Use the Descriptive Word Index. Look up terms to find Topics and Key Numbers that have relevant abstracts.
Use the topical approach.
Choose one or more Topics that are likely to address the issue you are researching. Read the scope notes to see whether your issue is covered. Skim the outline to find a Key Number or range of Key Numbers.