This example demonstrates Crossfilter's custom reduce functions by grouping a small dataset of car specifications by brand and calculating the average horsepower for each brand. Using a crossfilter dimension on car names, the code groups records by the brand portion of each name and applies a custom reduce that maintains running totals for total power and count, computing the average on each add/remove. The groups are then ordered by the resulting average and rendered as a simple text list showing each brand and its average power.