The demux object takes index of outlet from the right inlet. Then whatever data is sent to left inlet is then sent to proper outlet. Because the index has to be sent first (default will be 0), the bang is connected to index message first and then the list message. You can make clear the order, visually speaking, by using trigger object.
The data that is sent to outlet 0 or outlet 1 is a list of 2 numbers. Here, the number of outlets is 2 since that is the number of arguments.