I should have watched the video first before posting the above question.
It seems the offline mode is very basic, and the data has to be very static. I don't see that it would be possible to create an Invoice offline with product line items - there would have to be formula running in the form to calculate Invoice total, Item total, sales tax , discounts etc.- and also printing the invoice could be a complex process
So the offline feature is basically for offline form capture, something like a Credit Application form , Or data that is being captured in the field .... basically replaces a clip board and form scenario.
This feature should possibly be called "OFFLINE FORMS " this would be more meaningful description to the feature, an excellent feature to say he least (although a simple Task list or Diary app could work offline so i suppose its a little more than just Offline Forms)