Database - Not Live
Sonar is a huge database of product/keyword combinations. Sonar currently updates on a irregular basis.
If the respective product is relatively new on Amazon, it may not yet be part of the database.
Time of last update: End of April 2017
Only Well Ranking Keywords
Sonar only includes keywords for which the ASIN is ranking on the first page of the searchresults (or at least was ranked at the time of the last database update). The ASIN Reverse Search therefore offers a good opportunity to find out the keywords with which any given ASIN is generating most of its (organic) sales.