Once a user is enabled in a Full-Duplex channel, its webcam button on the toolbar - that until then is disable - turns into an active status. In this moment the user sends its audio and video. All the users in the conference room listen and see the enabled user.
PC & Mac: webcam button
Tablet & smartphone: webcam button
If you need to stop sending your video, press the button to deactivate it (in PC & Mac you can also use the main horizontal menu -> Channel -> Enable/disable webcam and microphone).
Please note that a click on the webcam button disable both webcam and microphone buttons at the same moment in PC and Mac. If you need to turn off only your video, press this button, then enable the microphone button.
You can turn off your video also if you have some connectivity problem (not enough bandwidth): in an only-audio mode you will use less connection.