Check it out at: http://classtechtips.com/2016/05/14/how-to-start-a-backchannel-in-your-classroom/
A backchannel provides a space for people to chat about a shared experience in the moment. Twitter is used as a backchannel during popular events like the Superbowl or the season finale of a favorite television. In this instance everyone tweets using a hashtag as they watch a live event and you can see what other people are saying.
In the classroom, a backchannel can be used for students to have conversations, share an idea, or pose questions about a topic. You could watch a BrainPOP videowith your class and pause every minute or so to have students jot down a question or respond to a quick prompt using a backchannel.