Tony Russell-Rose – Interaction Models for Faceted Search:

"Note that the facet values examined in the two-stage examples above are disjunctive (multi-select OR), e.g. the selection of a value for a facet such Make & Model does not preclude the selection of another value from the same facet. In this case, selecting multiple independent facet values has the effect of widening the search. However, if the facet values are conjunctive (multi-select AND), then the choice of which interaction model to apply is quite different. […] In this case, the only meaningful interaction model is the instant update, as this is the only approach which will ensure that facet values and the current result set stay in sync."

(Via Patrick Durusau.)