The Task Burndown report shows the number of tasks in each state at any particular time. So for example, you should see that the number of completed tasks increases while the number of open tasks decreases before the release.
To create the Task Burndown report go to the Reports view. Right-click 'Reports' or a report folder in the tree and select 'New | Report'. Select the template category 'Task Report'. Select the template 'Task Burndown' or 'Task Burndown By Priority'.
After giving the report a name and description you can finish. This will create the report which you can select to view.
You can customize the report according to which projects to report on and what date range to use as described in the section called “Customising a Report”.