You can check details in procd init
procd actually ignores the process specification "/etc/init.d/rcS" (there is no such a script!) in "/etc/inittab".
The implementation is in rcS.c:
int rcS(char *pattern, char *param, void (*q_empty)(struct runqueue *))
{
    runqueue_init(&q);
    q.empty_cb = q_empty;
    q.max_running_tasks = 1;

    return _rc(&q, "/etc/rc.d", pattern, "*", param);
}

which is called by "runrc" function in inittab.c
static void runrc(struct init_action *a)
{
    if (!a->argv[1] || !a->argv[2]) {
        ERROR("valid format is rcS <S|K> <param>\n");
        return;
    }

    /* proceed even if no init or shutdown scripts run */
    if (rcS(a->argv[1], a->argv[2], rcdone))
        rcdone(NULL);
}