What is the field type you have set up in DataAssist.
Here is the problem, when you don't check a checkbox it doesn't return a value, when you do it returns the value specified.
Your technique to have a checkbox to disable or to enable depending on the start value just doesn't fit the paradigm very well. It would be much easier to have a single checkbox "check to enable or uncheck to disable".
Trying to get what you are doing to work would require more hand coding where the field is updated in the server code as well.