This came out of a discussion on the java-user list with subject
"Faceted search in OR": http://markmail.org/thread/jmnq6z2x7ayzci5k
The basic idea is to count "near misses" during collection, ie
documents that matched the main query and also all except one of the
drill down filters.
Drill sideways makes for a very nice faceted search UI because you
don't "lose" the facet counts after drilling in. Eg maybe you do a
search for "cameras", and you see facets for the manufacturer, so you
drill into "Nikon".
With drill sideways, even after drilling down, you'll still get the
counts for all the other brands, where each count tells you how many
hits you'd get if you changed to a different manufacturer.
This becomes more fun if you add further drill-downs, eg maybe I next drill
down into Resolution=10 megapixels", and then I can see how many 10
megapixel cameras all other manufacturers, and what other resolutions
Nikon cameras offer.