Users X and Y are working on the same task called 'Design Website'.
Since the task is active, they both will have it on their In Progress tab, My Assignments.
As user X finishes his part, he clicks 'I'm Done'. This will have a three-fold effects:
1) The task disappears from user X's In progress tab.
2) In Design Website task's details, in Assignees section, user X will have the 'work is done' checkbox selected automatically.
3) The task's status will not change.
User Y will, as before, have the task on his In progress tab, and the task will not get Completed status until user Y clicks 'I'm Done'.