Motion Detection is to detect movement outside the vehicle, say a shopping cart or car hits the vehicle while its parked. The G-Sensor is for the about of G force or impact of the collision and where the camera will automatically tag and save the video clip. The higher the G the higher the impact will have to be to initiate the tag or save point. I keep it all at 2G, so it tags anything even light bumps. Keep in mind bumps and pot holes in the road will initiate tags. even if the sensor is off you still can hit the save button on the back of the device after an accident happens ( to you or someone else). Hope this helps.