If a player shoots with a gun on the same tile where the entity lays, the entitiy triggers. This tile must be a wall, otherwise, this entity will never trigger. The player also must fulfil the condition set in the Triggered by setting.
If a Trigger_Hit entity gets triggered itself, it disables/enables again. A disabled Trigger_Hit entity does not trigger if it got hit. At round start, all Trigger_Hit entities are enabled.