You may have noticed that in the Settings > Controls > “Control mapping” screen, one of the actions to map a key or button to is called “Rewind”. It’s between Pause and Save State on the list, and it is set to
kbd.Backspace by default. And if you press the key bound to this action while playing a game, the emulator will just give an error: “No rewind saves states available.” How can you get this feature to work?
The answer is to go to Settings > System and look in the Emulation section. There is an option “Rewind snapshot frequency (mem hog)”. By default it is set to Off (0), which is why the Rewind key does nothing. Changing the option to something other than 0 will enable the rewind feature, which periodically makes save states that you can rewind to.
From PPSSPP 1.15, this is easy - it's the number of seconds between each rewind snapshot. Set it to 0 to turn off the feature (and save the RAM it requires).
In older versions, as its editing interface shows, you can set a frame count from 0 to 1800, where 0 means that the feature is off. PSP games usually run at either 30 or 60 frames per second (you can tell by going to Settings > Graphics > “Overlay information” section > “Show FPS counter” and setting it to FPS). So if you set the option to 150 frames and then play a 30-FPS game, a save state will automatically be saved every 5 seconds.
If you make a mistake while playing, you can press the Rewind key, and the emulator will load the last automatic save state it made. In the example above, this save state could have been made anywhere from 0 to 5 seconds ago. After pressing the key, you can try that section of the game again. If your mistake was further back than one rewind frequency, you can press Rewind repeatedly – each quick press takes you back to the previous automatic save state.
To find out or change what key or button it is, go into Controls / Control Mapping and check or set it up.
Enabling this feature causes PPSSPP to use up quite a bit more memory than normal, can be several hundred megabytes, depending on the game. So set the rewind snapshot interval back to 0 when you’re not using the rewind feature. And note that you will have to quit and relaunch the emulator to free the memory that was used when you enabled the feature.
The number of rewindable states the emulator saves is currently 20. That number means you can press Rewind up to 20 times in a row to go back, and after that, you can't go back any further.