Starcraft 2 introduced replay rewinding which I think is even more interesting.
I guess that it loads a replay at the time you're watching it simply by replaying the game with the help of actions and at the same time it stores states of the game and then if you rewind (you can't continuously rewind like a VCR, you pick a state in the past and you go on from there) it loads that state.
It sounds so easy when you write it like this but I'm sure it's a lot harder than it sounds.
I swear SC1 had rewinding in its replays. Or at least jumping to the past. And from what I remember, SC1 replays recorded all user inputs, timing data, as well as w/e the unit AI (pathfinding and auto attack) did. This would cause some problems across patches which would make previous replays glitchy since something about the unit stats would change. For example, if a unit had 45 hp and then upgraded to 50hp in a patch and the replay had a unit kill the 45 hp unit and immediately leave, then in the patched replay, you would just have a 5hp wounded unit left standing there.
There were all sorts of tools for analyzing replays. They did contain ALL user inputs, so you could map a players APM second to second through the entire game, and you could also break out what percentage of their clicking was spam, and what were actual commands.
Starcraft 2 introduced replay rewinding which I think is even more interesting.
I guess that it loads a replay at the time you're watching it simply by replaying the game with the help of actions and at the same time it stores states of the game and then if you rewind (you can't continuously rewind like a VCR, you pick a state in the past and you go on from there) it loads that state.
It sounds so easy when you write it like this but I'm sure it's a lot harder than it sounds.