I believe de-judder is when it displays each frame the same number of times instead of doing 3:2 pulldown. On a 120hz TV, it would display each of the 24 frames 5 times, so with each letter representing a frame you'd have A,A,A,A,A, then B,B,B,B,B and so on. The result is the appearance of displaying 24 frames per second. On "regular" TVs, it has to fit into 60hz, so it will go like A,A,A, then B,B, then C,C,C, then D,D, and so on. The result is judder caused by the uneven cadence.
This effect people are talking about, which is common on 120hz TVs but optional, derives fake frames by looking at two successive frames and inserting a frame for the difference between them. This effect is created solely by the TV, not the source, but some people have been led to believe it is due to the BD format itself, because it often happens to be demonstrated along with BD playback. The same thing can be used with DVD, or even games (although with games you'd get input lag, since it has to wait for the next frame to know what to display in between).