While watching realtime contents such as presentations and TV shows, people are attracted to backchannel communication, which is now supported by various CMC systems over the Internet. However, communication tends to be filled up with simple reactions such as excitements and agreements due to high cognitive load to track content and communication concurrently. Although simple reactions may help communication through highlighting common interests and fostering a sense of unity, it bothers participants trying to deepen discussions. We propose three design principles to overcome the problem: 1) simplify input and recognition of the relation between content and communication 2) provide input methods for different content-communication engagement levels, and 3) visualize communication to enable coexistence of deep discussion and simple reaction. On-Air Forum is a chat system developed based on the principle, which is used as a backchannel chat at an academic conference in Japan (WISS). For users fully concentrated on the content, the system provides eyes-free input to share excitements to the content (excite message). Users can react to others without typing messages through agree/disagree buttons and instant QA. Reactions collected are used to help deepening of discussions through visualization.