The purpose of this app is moving all sub-tasks from one Parent issue to another Parent issue.
After installation, you will see an additional option, Move All Sub-tasks under More on the Issue details view.
See link circled in green below
When you click the Move All Sub-tasks link, you will be directed to a new page to start the move operation.
First, you will be asked to tick any sub-tasks you want to move.
Then, you select new parent issue where the sub-tasks are to be moved.
Then, click the blue Move button.
Alternatively, you can filter your sub-tasks to be moved by entering JQL.