The Backchannel is a hidden channel of communication in meetings and events. Brakes the rules of one direction communication. It´s like a radio just for the meeting where people can read,write and leasing at the same time.
The key point of the massive use is related to the real time interaction with the audience. If you are a speaker, i should recommend you to take a few minutes and check this slide share. If you don´t care , please stop reading here. and take this tip for the next event. do a research on Twitter when you finish your speech, maybe you can find something like "This speaker sucks" .. if yes don´t panic.. just comeback to the presentation and press de button.