You can use your mouse to drag-select a section from the “Overview” (on the left side of the heatmap page). The section that you highlight will appear in the “Focus View”.
Since gene set analysis is performed on the genes in the Focus View, this allows you to visually identify clusters of genes with distinct expression patterns, view them in more detail in the focus view, and try to understand which biological processes they represent using enrichment analysis.
Tips: apply different clustering methods to genes (cluster genes) or samples (cluster samples) to discover interesting patterns