It’s called raspidisp (for Raspberry Pi) and can turn a given display output into a Video4Linux-compliant capture device (like a camera). The UV4L Streaming Server itself makes use of it in a new functionality which allows to both mirror the RPi display and send keyboard or mouse inputs to the Rpi from a web page in the PC browser (it works with smarpthones too, except the keyboard is not supported at the moment) – plugin-free! As UV4L can capture audio too, adding an audio track (e.g. from HDMI-out) is left as an exercise to the user. Below is a DEMO.
It’s quite easy to try this out by yourself as does not require any special configuration. Please refer to the installation page for the instructions.