Making APIs Faster: Introducing Partial Response and Partial Update – The official Google Code blog
It’s easy to understand the benefit of partial response and partial update. Imagine that you are writing a new Android calendar widget, and you want to display the time and title of the recently changed events on your Google Calendar. With the old Calendar Data API, you would request your calendar’s events feed and receive a large amount of information in response — including lots of extra data like the attendee list and the event description.