A Condition involves the examination of state. States change due to events occurring. Every time a rule encounters a Condition it evaluates it by examining the state of its elements. A Required Expression is evaluated only when the state of one of its elements changes. When the Required Expression becomes false, the Event Subscriptions for the triggers are removed -- the rule won't trigger from any of these events. When the Required Expression becomes true, those Event Subscriptions are created, allowing the rule to be triggered again. The Required Expression is not evaluated when the rule is triggered.
This might be helpful: Introduction to Automation
