Drill-down queries find "expansions" or longer phrases incorporating query terms. The results indicate the number and percentage of occurrences of each phrase represented by specific longer phrases. They also help detect the boundaries of recurrent phrases.
By default drill-down queries return expansions both right and
left of the query term. To restrict the direction, insert start / end of phrase
Drill-down sums all counts of a given string, ignoring PoS tags. Consequently frequencies may be greater than those reported by other query types where the same sequence of words received variant taggings.
Fetching explanation of Regular Expressions