I struggled with the same issue, but was unable to find a suitable answer. As a work around I created a graph to show the amount of tasks and their status in the current sprint. A spike would appear in the category 'new' (for PBI's) or 'to do' (for tasks). For clarity I separated the PBI's and bugs (which I see as a high level) and the tasks (which I see as a low level).
If you want to do this:
1. Create a shared query which:
- work item type = Task
- iteration path = @currentInteration
2. Set the correct columns as output
- state
- ID
3. Create a line graph based on the query
- Line chart
- group by state
- count of work items
- rolling period: choose one the options
- sort on what you want
4. Add the graph to the dashboard (that is why you need to create a shared query)
For this works fine (or actually as best as possible with my current understanding of TFS)
Hope this helps