Files in folder ".ipynb_checkpoints" are nessasary for the extension, do not remove or delete them.
Generally, you do not need the "begin" and "end" statement in block, because the extension would automatically wrap every block with them, except one containing "load xxx" or a function/procedure
The interrupt button can only stop the timing, not really stop the running process.
The restart button will clear all output and restart the running kernel.
How to fully reset the extension? Just delete the folder: .ipynb_checkpoints and click restart button.