The normal failover mechanism in cgminer will allow some work to go to the backup pools if there is a delay in getting work from the primary pool you have set. This can be as much as 10% if the pool has latency issues. It then only fails to the backup pool if the primary pool stops giving work for 1 minute. failover-only does not change the failover mechanism, but it does not allow work to go to the backup pools unless the primary pool has failed completely for at least a minute. It then will switch to the backup pool. If cgminer considers a pool dead or down, it tries to contact that pool 1 minutely.