The F11 Key Explained

The image above shows the display of a Sudoku puzzle at the very moment that a breakpoint has been set. These are set automatically whenever you choose one of two possible numbers in a square. The message in the oval will disappear in about one second.
As you continue to solve the puzzle, if you wish to return to the state it was in when the breakpoint was set, simply hit the backslash key...the one on the upper right of the keyboard layout just to the left of the backspace key...and the puzzle will revert to that point. This is useful when you have made a guess on which of two alternative paths to follow. If you end up at a "dead end"...which occurs when one or more squares in the puzzle will not accept ANY number, then you can go back and try the other number, which must now work...at least until you have to make another guess. You should NEVER enter a guess when there is more than two possibilities. For one thing, it is not necessary. For another, it makes it extremely unlikely that you will have chosen the correct number. Then, you can set yet another breakpoint. That way, you can gradually advance through a difficult puzzle.
If, however, you turn on the F11 function all the breakpoints that have been set will be illustrated. They will be green if that is the first time you have visited that breakpoint, and red if you have already tried the second of the two possible numbers. Then, if you hit the backslash key, not only will you be returned to the last previous breakpoint, but the program will automatically insert the second number. If, the second number has already been tried, then it will automatically go back to the previous breakpoint...if there is one...or to the beginning of the puzzle.
If you want to go back to the very start of the puzzle, hit one of the delete keys. One is underneath the numbers in the keypad, the other is just to the right of the PageDown key to the upper left of the keypad. Arrangements may differ on some keyboards. Finally, if you simply want to go back, one move at a time, to ANY point in the process after the beginning, you can hit the backspace key, which is usually placed at the upper right of the keyboard with a left pointing arrow displayed on it.