Interlaced CRT scanning. This trick displays a complete television image as a set of two interleaved frames, first even then odd rows for instance. This scheme allows CRT TV images to appear to be more responsive to motion and with less flicker, even at relatively low frame rates such as the NTSC 29.97 frames/sec broadcast standard.
The need for interlacing has long vanished in the LCD display, digital streaming and digital broadcast world, supported as a legacy standard if supported at all.