Subtasks allow you to make lists within lists for those times when creating a whole new queue is overkill.
To make an existing item a subtask click the arrow or hit the right arrow key.
Click the arrow or hit the left arrow key to move the subtask back out.
You can create as many levels of subtasks as necessary.
You can drag tasks to any level of the hierarchy by clicking on the item's number and dragging.
To constrain dragging to the current level click anywhere on the item (except on the number) and drag.
To expand or collapse subtasks for an item just click on the item's number.
To expand or collapse all subtasks in the entire queue you can choose that option from the dropdown menu.
You mark subtasks complete the same as regular tasks - by clicking the checkmark. As a convenience, if you click the checkmark on an item that has subtasks, all of those subtasks will also be marked complete.
Completed subtasks do not show up in the queue's Completed tab until the top level parent task is also complete. To view completed subtasks while a parent item is still open click the magnifying glass.
This will bring up the Item Overview which shows the open and completed tasks all together.
When you view a Smart Queue or one for a specific tag you may see subtasks in the list without their parent (because the parent did not match the filter of the Smart Queue). As a point of reference, you can click the parent arrow on subtasks to bring up the Item Overview.
If you delete a task that has subtasks, all of the subtasks will also be deleted. This action cannot be undone. For safety, you will be alerted with a warning message before the delete is carried out.