system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what is rollback in fighting games and how come it makes it feel like there isn’t really any lag?
gpt
When you play online it takes time for control inputs to get from player A to player B and vice versa. In a rollback scenario, player B's computer is constantly trying to predict what input it will receive from player A and animates the game based on the predicted inputs. When correct, it appears lagless. When incorrect, the game basically skips a small amount to correct the mistake.

So for example:

Payer A presses X (punch) at time 0.00s

Player B's computer predicts player A pressed Y (kick) at time 0.00s. So now player B's computer starts to animate a kick.

At time 0.1s B receives the command player A pressed X at time 0.00. Player B's computer "rolls back" the game 0.1 seconds, works out what would have happened if a punch was thrown over those 0.1 seconds, and then skips forward again to adjust the game for the actual input. From player B's point of view, half way through a kick, it changed to a punch. However player B was able to press block, which is equally valid on a punch or a kick, so in this case the incorrect cue from the prediction algorithm was still helpful. 

When lag is low (say 30ms) this isn't noticeable, but when high, the quality of the predictions becomes important and artefacts like teleportation become apparent.

To further improve things some games include a small amount of input lag, say 20ms. This lag is always there, and it's always constant so you're used to it. This 20ms input lag gives the game a little window for the game to "catch up" in during a rollback event.