Jupyter creates a checkpoint file every time you create a .ipynb file, and then updates a checkpoint file every time you manually save your progress for the initial .ipynb . Manual saving is what is done when the file is saved by clicking the "Save and breakpoint" button:
Auto save, on the other hand, only updates the source .ipynb file, not the checkpoint file.
When you return from a .ipynb source file to a previously saved checkpoint using the " .ipynb to Checkpoint" button, the checkpoint file is accessed and opened inside Jupyter.
As an additional note, the checkpoint file is located in a hidden folder named .ipynb_checkpoints . This folder is in the same folder as the source .ipynb file.