iWitness is one of those must have apps that you need for personal safety. iWitness should be one item in your overall plan to keep safe. It will not prevent bad events, but it could actually save your life.
The way it works is you 'arm' the app before you enter a 'potentially threatening environment'. For example, if you are having to walk to your car late at night, you would arm the app before you walk out the door.