For those of you who are new to movie codecs, here is some info that might be useful:

roughly, videos are not streams of images, one after the other (well motion jpeg is, but ignore that) They are "key frames" ie full images, then a set of blocks with some vectors that move those blocks around to make a moving image. So you'll see something like b, i and p frames, each have a different role for making either a full image from which the next frames re-constructed, or the blocks that make the in-between frames.