 This illustration shows the board after you have invoked the F8 function, either by hitting the function key or clicking on the F8 icon on the Sudoku board. Notice that each number appears on the board in the position corresponding to its place in a 3x3 array which looks like  when completely filled. This display is similar to that for the F7 function, except that the numbers are red instead of black. Note that some squares have only one number in them, making it simple to locate the other one of the pair. Other squares have more. For example , the fourth sqare in the first row has the numbers 4, 5 and 6 in it. The 4 corresponds both to the other 4 in the first row...OR..the other one in the fourth column...OR..the other 4 in its box. The 5 corresponds to the other 5 in its colum as well as the other 5 in its box. This is true because all these possiblities are true. The F9 function, explained below, makes this clearer and easier to see.
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This is how the Sudoku board will look after the F9 function is invoked the first time. This means that in the box at the upper left, ones can be placed ONLY in those two squares. If the function key is hit again, the green squares will disappear. The third time it is hit, another pair of green squares will show up somewhere. You can repeat this until all pairs have been seen, at which time you will be back to the first pair of ones. Note that placing a number in either of these squares will alter the subsequent possibilities.