Flicker is a plug introduces flashing/flickering brightness into your clip. if you choose a low frequency (e.g. twice a second), it looks as if the active part of clip is flashing (highlighted) like a cursor. on higher frequencies, it looks like those old super-8 films when using an old projector.

Alternating is a change in the intensity of light, typically seen in a range between a few Hertz and 60 Hz when displaying images such as pictures of text. In an existing projector, if a flicker phenomenon, that is, flickering of a screen image, occurs, then either the lamp is exchanged or the input power to the lamp is increased so that the luminance of the lamp becomes equal to or higher than a preset value. However, the existing projector has a problem in that the life of the lamp upon low input power driving when a flicker phenomenon occurs may not be assured sufficiently.

The brighter the room, the greater the frequency required to eliminate flicker. Movie theaters are dark, and the projector doubling to 48 Hz is sufficient. TV is often viewed in lit rooms, which is why 60 half frames is required (see NTSC). The highest HDTV rate is 60 full frames per second.

Flicker Fusion Threshold:

Most artificially created "moving" images we watch (film, television etc.) are not actually moving but are a series of static images. The frequency at which flicker becomes invisible is called the flicker fusion threshold, and is dependent on the level of illumination. Generally, the frame rate of 16 frames per second (frame/s) is regarded as the lowest frequency at which continuous motion is perceived by humans. (Interestingly this threshold varies across different species; a higher proportion of rod cells in the retina will create a higher threshold level.)

The average flicker fusion threshold for human beings is 16 Hertz for 50% of test subjects, in other word if a light flashes on and off 16 times per second, 50% of people will see a steady light. This is why we have 50 and 60Hz refresh rates, where almost all people will perceive a steady state rather than flickering.

It's worth noting that we are more susceptable to this flicker effect in our peripheral vision than in our foveal vision, which is why we see the rainbow effect when we flick our eyes back an forth when watching a DLP projector.