The "history" is the list of completed tasks in a project. To access it, please access a project and click on the "Get completed tasks" link at the bottom of your task list in this project. Each task there can be recovered by un-checking the box next to it.
Sub-tasks for a completed main task are not showing up in history. The main task is showing. I wanted to get back the sub-tasks, which were automatically completed (checked-off) when the main task was completed.
Are you referring to the list of completed tasks inside a project? It should show the completed subtasks too: the circular icon in the end of the task list (when you are in the project view). Can you see your completed subtasks there?