Master Task Roll-up and Roll-down Button
A button that allows the user to roll-down all the subtasks with one click rather than having to individually click all subtasks.
I think what is being asked for here is an expand all feature. Do you can see every sub task in the project without having to expend each task individually. It's useful to look through the complete project occasionally to check nothing has been missed.
I'm not exactly sure what you are asking, you can complete all the subtasks by clicking the parent task.