If you register sixtyforce, autosaving and game freezes will be enabled. How they work can be a little confusing, so here's an attempt to explain things more clearly:
Autosaving happens when a game tries to write a save game to the cartridge. The save game is instead automatically saved to your computer, so the next time you open the game, your progress will still be there.
Note: This only works for games that had saving built-in originally. To use autosaving, you may have to choose some kind of 'save' option in the game itself, but other than that you don't have to do anything.
A game freeze is a special feature to sixtyforce — it will "freeze" the game at the exact point in time that you save it. When you later open the game freeze, the emulator will restart to the moment you saved the freeze. So you could save a game freeze right before a tricky jump, then reload the game freeze until you make the jump. Because a game freeze is part of the emulator, it should work for any game at any time.
To use a game freeze, just choose 'Save Game Freeze' from the 'File' menu. Save the file anywhere you like and later you can reopen the file to restore the game.
Using Both at the Same Time
If you "freeze" a game, part of what gets frozen and restored is any save data that was saved "to the cartridge" at that point in time. This means that if you opened a game freeze with autosaving enabled, the game freeze would reset your autosave. Because of this, sixtyforce asks you if you really want to do this when you open a game freeze:
The safe choice is 'Disable Autosave' because this will prevent the game freeze from overwriting your autosave, but if you choose 'Disable Autosave', when you're done make sure you use another game freeze to save your progress!