Hi Ruben,
From desktop, you can share your entire screen. If you happen to have several displays connected, you can select which display to share.
You cannot share individual applications. However - maximizing your app would mostly do the trick. To share e.g. a PPT presentation, I start the slideshow view and *then* start Proximity sharing using the keyboard shortcut (Alt-F12)
From mobile; we do comparison between all snapshots (1-2 per second) and only transmit those with "significant difference". We always try to optimize the algorithm for "significant difference" so it makes sense. In my experience, it might miss a single letter added to a word document, but usually detects well enough for additional bullet points/builds in a presentation.
If you show a video (youtube playback, for example), there are usually big enough differences in the image to trigger a new slide. So - the go-back-to-previous is obviously not useful while in video playback, and is optimized for typical presentations.
Hope this helps,
-Henrik
