UpdateWithScript updates a document based on a script provided. The operation gets the document (collocated with the shard) from the index, runs the script (with optional script language and parameters), and index back the result (also allows to delete, or ignore the operation). It uses versioning to make sure no updates have happened during the "get" and "reindex". (available from 0.19 onwards).
Note, this operation still means full reindex of the document, it just removes some network roundtrips and reduces chances of version conflicts between the get and the index. The _source field need to be enabled for this feature to work. http://www.elasticsearch.org/guide/reference/api/update.html