If hl.q parameter is set, HighlightComponent uses it rather than q.
You search "PC" with highlight and facet capability:
You get a lot of results with snippets (term "PC" highlighted in desc field). Then you click a link "maker:DELL(50)" to narrow the result:
You'll get narrowed result with term "PC" highlighted snippets. But, sometimes I'd like to see "DELL" to be highlighted as well, because I clicked "DELL". In this case, hl.q can be used:
Note that hl.requireFieldMatch should be false (false is default) in this scenario.