Force is a sudoku solving algorithm. Basically it resorts to
trying a number, if it doesn't work, try another number, and repeating
until you solve the puzzle. It is best suited
for computer use. Performing Brute Force manually would be very tedious. Its something to resort to when all else fails.
More practically, Brute Force is useful for verifying puzzle validity. If Brute Force can't find a solution, then the puzzle in insolvable. Brute Force can also be used to prove that a puzzle has only one unique solution. It does this by running Brute Force twice, once foreward and once backwards and comparing results.
Brute Force looks at one cell at a time. It starts in the upper leftmost cell, then moves left to right for each row until reaching the last cell on the bottom right of the puzzle.
Remember that GIVENS are fixed cells present in the puzzle grid. GIVENS are ignored. The algorithm is then:
Brute Force method still starts at the top left and progesses to the bottom right.
The two solutions should be identical. If not, then the puzzle has multiple solutions.