This trigger causes any units that have a guard waypoint, to come to the area the trigger is in when the triggers conditions are met.
So for example, you could have a "Guarded by" triger that covers a section of woods activated when West is detected, a group of East soldiers somewhere that have a Guard WP. If West units are detected in the woods the trigger covers, the East units would move to the area (actually center) of the trigger and begin looking for the West guys.