You change the state of the horn to be on, but I'd imagine you might want the horn to automatically sound for a predefined length of time. Then the mapping is less obvious. Agreed that it doesn't really matter much in Tesla's case anyway.
You're still changing its state. If you have one horn you can do whatever you want with it, just like a normal person would. What the client does with a resource is only limited by the API, REST doesn't care what the messages contain. The semantic of the message is up to the API to resolve, you could encode that "stay on for 10 seconds" however you wanted.
