[manasecret]

Too hard to understand? Maybe I'm reading too many science books lately...

Here's I think a simpler explanation pulled from a thread at avsforum:

Anyway, the purpose of 120Hz is to get rid of 3:2 pulldown, when watching movies in 24 frames per second.

When a movie runs at 24fps, a TV running at 60Hz can't accurately display the film perfectly. This is because 60 is not divisible by 24. As a result, the TV will show one frame for three cycles, then the next frame for two cycles, and continues to alternate between 3s and 2s. The result is 24 frames per second, but every "odd" frame is displayed for 50% longer than every "even" frame. Doesn't seem like a big deal, but in fast moving sequences, you will notice a vibrating effect on the screen, otherwise known as "judder".

120Hz TV's don't suffer from this problem because 120 is divisible by 24. In other words, each frame is flashed for the exact same amount of time (five cycles). Instead of 3:2:3:2:3:2..., you get 5:5:5:5:5:5. The result is smoother, crisper motion when watching movies in 24 frames per second.

Auto Motion Plus, Motionflow, Cinemotion, whatever... is an additional technology that processes artificial frames by interpolating the images in the "before" and "after" frames, and then adding the frames in order to enhance the frames per second of the picture. The result is really smooth motion, which looks unnatural and unpleasant to some. Apparently, none of the companies can execute this perfectly, resulting in "artifacts" like the "triple ball effect" and stuttering, when the processor can't handle the processing and falls behind. If you don't like this feature, you can turn it off instead of sending your TV back to the dealer. 120Hz is still awesome.