Here is what happens. The presenter is sharing their entire screen, not a document nor an application. The mobile app, I'm using an iPhone, is looking for differences in redraws of the screen. When it notices a large enough change from one redraw to the next, it takes a snapshot of the screen, and then advances the "slide." This gives you the illusion that you are actually in control of a slide deck. What's really happening however, is you are moving through a history of screen changes, or screenshots.
Typing text into a text box just like on this forum, is not enough to trigger frequent redraws and thus snapshots. So, the users on mobile watching me type this are not seeing the text entered in real time. Instead they see infrequent screen updates of large chunks of text at once. That's different for sharing content over proximity to the video endpoints itself. I.e., My SX10 is showing me type this in near real-time. Small lag, but certainly nothing like how the mobile app is working.
You can cause a redraw event to ocurr on a mobile device by simply pressing CTRL+A, which will highlight all of the text on the screen, and trigger an event.
Obviously, this feature was not intended to watch people type up replies to emails and the like, so when the presenter is showing a slide deck, the illusion works pretty well.
Also, there didn't seem to be anything stopping me from screenshoting the presenters desktop during sharing, so if you're sharing sensitive content for which you wish to control distribution, maybe be cautious.