When the streaming is going live, you should see a small LIVE sign with a Play Button. You may have to refresh the page to make this happen.

The stream will be up shortly before the start time, just playing music and showing a live countdown to the start of the class.

To the right of the streamed image, there is a Full Screen button, if you want a bigger picture. 

Find some space, grab some water, and enjoy. Remember to exercise within your own limits.