This feature works only when Ogame Bot is active. When it is active, Ogame Bot refreshes the page in the intervals you predefined and checks if there is an attack on your planet or moons. If there is an attack happening, it relocates your fleet to the coordinates you predefined and in the fleet settings section in the Ogame Bot, with the mission you selected.

Ogame Bot waits for twice the time of waiting period used in message sending before relocating the fleet. For example, if the waiting period for message sending is 3 minutes, then your fleet will be re-located after 6 minutes waited. It makes no difference if the message sending feature is active or passive.