I also have a ‘Watch’ column.
This is for tasks that are done by someone else. It’s useful when you are working with someone else on the same task in iterations. When I move a task from ‘Doing’ to ‘Watch’ I add to the name of the task the name of the person I’m waiting for and a due date.
For example, I have to complete a project description and a collegue of mine, Andrew, has to review it. I have a task named ‘Complete project description’. When I finish my part, I send the project description to Andrew. I move the task from ‘Doing’ to ‘Waiting’ and it becomes ‘Complete project description — Andrew’. I also add a due date so that it lights up when it’s time to ping Andrew about the status of the task.
When Andrew gets back with his comments, and he is not happy, I move the task to ‘Doing’ again. This may go on until the task is finally done.