most compressed video formats encode frames by tracking differences from one frame to the next, which means you can only construct complete frames in the forward direction. also, indexing, pointers and offsets only point into the future, making seeking and reading in reverse pretty much impossible.
but you can still seek to 10%, 20% etc. by pressing 1, 2, etc. respectively.
mplayer will let you jump backwards 10 seconds, but also cannot play movies in reverse.
Most encoding schemes that use frame to frame differences put in full frames every now and then (like once a second) so that if anything gets off, it will fix itself when the next full frame shows. This should allow some sort of reverse play (albeit jumpy).
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