Renegotiation means using pre-created peer connections to add additional streams by renegotiating offer/answer session descriptions.
In this demo; at first time; only data connection is opened. Later on, same peer connections are used to renegotiate audio, video or screen streams.
It is runtime insertion of streams!
Remeber, it is a testing demo for RTCMultiConnection-v1.4.js library. Documentation available here.