Alerting not work

I set the rule like that: evaluate every 5s for 5s, and condition as exist any value above 0.
expecting to fire alert after 5s pending, but alerting never fired.

I have dug into the document for all day, it seems that the rule as “evaluate 5s for 5s” means Grafana calculate the condition every 5s, if the condition is true, the state will turn to pending, then after 5s, the state will turn to alerting.