De-interlacing basically removes every second line, you can choose between removing lines 1,3,5,7, etc. (the odd lines) or the even lines, 2,4,6,8.. To make up for the lost lines the computer can either duplicate the remaining lines or make them up (interpolate). I normally use the interpolate option and just experiment to see which field I'll keep. As the resolution suffers I only use de-interlace when the image absolutely needs it. Notice that the right hand ball is in different positions depending on what field you keep.