Visualization speed:
This tool uses recursion and backtracking to solve any Sudoku puzzle. The algorithm starts by trying every number from 1 to 9 in the first open cell. If a number is valid (i.e., it doesn't violate the rules of Sudoku), it moves on to the next cell and, again, tries every number from 1 to 9. If a dead end is reached (no valid number can be put in a cell), the algorithm backtracks to the previous cell and tries the next number.
Try putting the visualization on different speeds to see this process in action. If you'd like to finish up a puzzle while the visualization is running, select the "Instant" visualization speed and the puzzle will be finished.