DB v1.6.2 and mysqli, trouble finding the usage for property num_rows

I am looking into the DB package for mysqli for PHP5 and I have a question about the folling lines of code in the end of the simpleQuery method:

        $result = @mysqli_query($this->connection, $query);
        if (!$result) {
            return $this->mysqlRaiseError();
        }

# this next block is still sketchy..

        if (is_object($result)) {
            $numrows = $this->numrows($result)
            if (is_object($numrows)) {
                return $numrows;
            }

# need to come up with different means for next line # since $result is object (int)$result won't fly...

            $this->num_rows[(int)$result] = $numrows;
            return $result;
        }

mysqli_query may return a result object which is taken care of, but what is the num_rows property used for?? I searched for num_rows and I only found it in the freeResult() method.