This feature turns IO Ninja into a universal proxy. You can redirect data from Serial into TCP, or from TCP to SSH, or write an incoming stream to a file and so on.
Just start two sessions (they can be of completely different nature), link them, and all the data received on one end will be transmitted to the other.
When two sessions are linked, the transmit pane is hidden: transmitted data is coming from the other side of the link (which is bidirectional). You still maintain the full control over the session state.