When you configure the partner, you select the file listener to trigger the inbound or outbound flow. When you save the partner, the partner subscribes to the file listener. The file listener triggers the flow according to the rules defined in the file listener.
For example, you configure a file listener to listen for new files arriving in a folder and then configure the partner to use the file listener for inbound flows. When the partner puts files into the specified folder, the file listener triggers the inbound flow.