1. Press shift while clicking the green flag (Turbo Mode).
2. To enter a puzzle, press the number keys to switch the number in use (0 to clear spaces). Click a square to set its value to the number in use. It is highly recommended that you press save to get the save code before solving. Press "Save" again to hide the popup.
3. If you choose to load a code, press "Load" and enter the save code.
4. To solve it quickly, press "Solve fast." To watch it do its thing, press "Solve visually."
5. If it says that the puzzle is impossible and you are absolutely certain it is indeed possible, then comment the code.
Here is a Youtube video explanation I made about the algorithm this project uses:
https://scratch.mit.edu/discuss/youtube/Q9IDGfrlFHc
Some codes you can load as a demo:
530070000600195000098000060800060003400803001700020006060000280000419005000080079
300801002201000604000000000809000106060000050702000409000509000904080705600107003
800406007000000400000000650509030780000070000048020103052000090001000000300902005
This sudoku is known as "the world's hardest sudoku":
800000000003600000070090200050007000000045700000100030001000068008500010090000400
Thanks to
@s_federici for the suggestion to have the user-entered numbers a different color from the calculated results.