This module adds an extra query type to apachesolr. This query type is made for the following purpose: we needed to make a facet with multi-select. The facet contained a taxonomy with parents and children (only one level deep). Due to the multiselect we configured the facet with the "OR" operator. When selecting a parent, and then a child this gave an unwanted result: it gave all the result for parent term id OR child term id.
This query type will only filter for children if one or more children are selected (and not on the parent term id anymore). This allows you to select multiple children also for other parent terms. Giving you the possibility to do a complex search.