Sprint backlog is a forecast of the functionality that will be built by the Development Team. It is one of the artifacts of the Scrum framework and is a real-time picture of the work that needs to be accomplished during the Sprint. I look at this as a plan for delivering the product, showing what items are being worked on and by whom. It solely belongs to the Development Team. The Sprint backlog is created during Sprint Planning and a highly visible artifact.
The Sprint backlog is a plan with enough detail that changes in progress can be understood in the Daily Scrum. Updates to the Sprint backlog are done by the Development Team at least once a day. For example, the development team updates the task board every day. This will help the Development Team inspect and adapt, inspect the sprint backlog and adapt the product increment. This makes visible all the work to meet the Sprint Goal.
Since the Development Team owns the sprint backlog, they can add/remove/modify items at any time so they can accomplish the Sprint Goal. The Product Owner indirectly modifies the Sprint Backlog by removing items selected from product backlog items. ScrumMaster can make suggestions for missing items to fulfill the Definition of Done. No one tells the Development Team how to turn Sprint Backlog into Increments of potentially releasable functionality.
Monitoring the Sprint Backlog
Total work remaining in the sprint backlog can be summed up at any point in time in the Sprint. The Development Team tracks the total work remaining at least for every daily Scrum. Since transparency is one of the pillars of empiricism- Please read my blog post on empiricism here http://agilekingdom.com/empiricism-the-foundation-of-scrum/
It is important that the Development Team keeps this information up to date, daily. By tracking the remaining work, the Development Team can manage progress of the Sprint Goal.
As new work is required, the Development Team adds it to the Sprint backlog. As work is completed, the estimated remaining work is completed every day! When the Development Team realizes that certain elements of work are deemed unnecessary, they are removed. Only the Development Team can change its Sprint Backlog during the Sprint.