Living Will, When Effective
Although a living will is valid at the moment it is properly executed and delivered to your attending physician, its instructions will only be followed when you are diagnosed as:  1. Incompetent and have an end-stage medical condition, or  2. Incompetent and permanently unconscious.

The diagnosis must be made by your attending physician, who is then required to promptly certify this diagnosis in writing.

For Example:
A principal who is mentally alert, but diagnosed with incurable cancer will not have the terms of his or her living will applied and will still retain control over personal medical treatment decisions.

See: Living Will, Who May Create; Living Will, Revoking; Living Will, Delivery