Sudoku Rectangular A-B-B-A B-A-A-B Rule

While playing Sudoku this morning I think I noticed something new (at least for me).

Every Sudoku puzzle is supposed to have a unique solution. It is not supposed to be possible to have a pair of numbers in a 3x3 square that can be switched and then have the corresponding pair in another 3x3 square to the side or above or below then just be switched, what I call an A-B, B-A -> B-A, A-B swap AND have those 4 cells form the corners of a rectangle

(see fig 4).

For instance, this morning in a puzzle I had deduced that the pair of numbers at pair position A-B was 1 and 6, (see fig 1) but didn't know which order. Shortly after that I figured out that B and A' were also 1 and 6, (see fig 2) in the middle column of the puzzle, but again I didn't have enough information at that time to make a final determination of the pair B and A'. Then I tried to figure out the position labeled ? in fig 3. At that point I realized that the cell at B' could not contain a 1 or 6.

Reason: If A & B and B' & A' can be swapped with 1 and 6 or 6 and 1, then there would not be a unique solution to the entire puzzle. The swapping of 1-6 & 6-1 to 6-1 & 1-6 would not break an otherwise completed Sudoku puzzle, as each row, column, and 3x3 grid would still contain 1 through 9

Fig 1

Fig 2

Fig 3

Fig 4 Illegal Combination

If my logic is incorrect, incomplete or unsound, please let me know.

This may prove to be a useful tool when trying to solve a Sudoku puzzle. I don't think I have seen this "trick" explained before.

Update 2009.04.08 - Note: All 4 corners of the rectangle have to be updateable cells for this rule to apply. Sudoku games begin with a certain amount of cells filled in, non-updateable. I noticed several examples in completed puzzles where rectangles do exist, but at least one of the corners was a non-updateable cell, to which this rule does not apply, hence the alternating A-B-B-A B-A-A-B in the title.