Servlet 3.1 API introduces a support for Non-Blocking IO, see this tutorial for more information.
The idea is that when the service code reads or writes the stream it does not block at all and only does a read or write action when a servlet container is ready to handle it effectively.
Early JAX-RS 2.1 API had a server-side prototype to help the JAX-RS service code utilize this Servlet 3.1 NIO features in a JAX-RS friendly way. Unfortunately that prototype was dropped from the final 2.1 API with the future major JAX-RS version expected to provide a much more complete and sophisticated NIO API.
CXF 3.2.0 has retained the implementation of the original JAX-RS 2.1 NIO API prototype and made it possible for the users to experiment with it.
Even Easier NIO Write