Over the next few weeks we will be rolling out Chat Replay, a new feature that brings the excitement of Twitch Chat to recorded videos. Now, when you watch a recorded stream or highlight, the chat from the live broadcast will show up right next to the video. Never again will you wonder which moments had the chat going wild or what messages your favorite broadcaster is responding to. With Chat Replay, you can fully immerse yourself in the Twitch viewing experience and relive the best moments from broadcasts exactly as they happened live.
Source: The Official Twitch Blog Introducing Chat Replay – The Official Twitch Blog