Excerpt of this description:
Servlet 3.0 allowed asynchronous request processing but only traditional I/O was permitted. This can restrict scalability of your applications.
This is resolved in Servet 3.1 (JSR 340, to be released as part Java EE 7) by adding event listeners - ReadListener and WriteListener interfaces. These are then registered using ServletInputStream.setReadListener and ServletOutputStream.setWriteListener. The listeners have callback methods that are invoked when the content is available to be read or can be written without blocking.
Thus, to sum up, Servlet 3.1 extends the possible use cases dealing with Non-blocking I/O.