Unlocked when you finish 2 victory laps and start your 3rd
Victory laps are only an option when you beat The Lamb in the Dark Room. A prompt will appear asking if you want to do it. You will start the game over again carrying over most of the items you got in your last run, but not all of them. You can not unlock any other achievements aside from Victory Lap related ones while you are in this mode, so don't risk ruining the run and dying by going after boss rush or Hush unless you are ridiculously overpowered. And make sure you always grab the Negative and go to the Dark Room, as you can't start another lap from the Chest.