Excerpt from eglibc-2.13/resolv/res_send.c, comments added to explain logic.

gistfile1.c

        /* 
         * ns == current nameserver index (eg. 1,2 or 3)
         * statp->retrans == retransmition time interval (this is defined in resolv.h)
         *
         * do a bitwise shift to the left to get the new timeout
         * if ns == 0 then this does nothing
         */
        int seconds = (statp->retrans << ns);
        if (ns > 0)
                // divide the time that was calculated after the bitwise shift by the nameserver count
                seconds /= statp->nscount;
        if (seconds <= 0)
                // seconds can't be smaller than 0, so set it to 1 in case it is smaller
                seconds = 1;
        bool single_request = (statp->options & RES_SNGLKUP) != 0;
        bool single_request_reopen = (statp->options & RES_SNGLKUPREOP) != 0;
        int save_gotsomewhere = *gotsomewhere;