How to do the flashing effect

[flashbeginner]

As demonstrated here:

Flashing (http://s7.invisionfree.com/Lac_viet)
The banner has this very nice flashing effect.

[Timmee_3Styler]

I believe it is having a motion tween but in the middle, the frame is blanked out..or faded out

[baby_annie]

A frame with white background, then a blank keyframe (dark background). It makes flashing effect.