Sam Laing describes what a ScrumMaster should and should not be doing during a sprint.
Let's consider this one: Don't update the burn down chart.
Why is it important that the ScrumMaster not update the burn down chart for the team?
The chart helps the team understand how well the user stories are flowing during the sprint... Will the team complete the sprint on time.
If the ScrumMaster updates the chart the team won't take ownership of the sprint's progress. When the team updates the chart, they'll be more focused on story progress and avoid trying to complete large batches of user stories at the end of sprint.