This dog has been trained to manipulate the situation by his actions. Naturally the dog would try to open the door by actively scratching or jumping at it.
Instead, this dog has been trained to sit ,without commands given, in order to make the door open. In fact, as the dog once gets up during the process, highly anticipating that the door will open, the trainer breaks up the process! The dog sits back down and the process of opening the door is then continued.
It clearly shows that the communication is perfect.
I sit- door opens. I don’t sit- door does not open. Seems very simple. Thus dogs as well as any other creatures, do what works. The challenge just is to communicate through exact timing, which behavior will work!
The reinforcer in this situation sure is to be released and allowed to go through the door!
Remember your mother used to say “eat your vegetables, then you can eat the cake”