1 - Enlarge the presentation
As an attendee, when a presenter invites you to join a Session and syncs their presentation, it is, by default, shared to a part of your screen.
The other part of your screen is always available and allows you to view the Session feed.
To view the presentation on a large screen, simply click on 
to "Enlarge the presentation"
2 - Minimize the presentation
You will just have to do the reverse path using the button 
to "minimize presentation"
