"Video Drive" (VD) and "Direct Drive" (DD) auto iris lenses are used to control the iris to keep a better image quality. A "Video Drive" lens is built in amplifier to convert the video signal from a camera for controlling the iris motor. A "Direct Drive" lens only includes the iris motor. So, this type of the lens is cheaper and the image quality depends on the output of the auto iris camera.
This mode is used to avoid the flickering on your monitor because of the frequency differences between your system and light. Some countries use two different frequencies, and some countries use NTSC system but their voltage frequency is 50Hz. This function is enabled to not only prevent the flickering on your monitor, but also fix the shutter to decrease color rolling on your camera. However, since the amount of light passing through the lens is decreased, the sensitivity is also decreased accordingly.
Typically, one object reveals different colors under different light conditions. For example, white paper becomes yellow when yellow light is projected on that paper. However, when ATW is enabled, white paper is still white through the lens and will not be affected under different light conditions. Your camera will tune the color difference automatically based on the specified white light.