How does the properties box in the message actuator work? I’ve tried numerous ways, but have no clue…
A message can have a body. If it is not set it is empty.
The property switch enables you to define a property. The message actuator will place the string representation of the current value of this property in the body of the message.
Unfortunately the body field can only be evaluated by a Python controller.
Have a look at the BGE Guide to Messages. It shows how to use it.