The teaching sites I usually recommend can be found
here and
here.
While I'm at it I'll try to explain how the empty rectangle move I posted above works as a chain as simply as possible - hopefully no Rocket Science. I've added a label e to r4c9 to make the explanation easier.
Suppose the 1 in cell a is
False (ie not True). Then the 1 in cell b would have to be
True (because there are only 2 1's in Row 7 and one of them must be True). In chain parlance this is called a Strong link. In general terms a Strong link exists between X and Y if you assume X is False and that forces Y to be True).
OK, so cell b is
True. Well, that means that the 3 cells marked c in Column 5 Box 5 are all
False, since there can only be one True 1 in Column 5. This inference is called a Weak link. A Weak link exists between X and Y if you assume X is True and that forces Y to be False.
Now here is the clever "Empty Rectangle" bit : Since all the cells marked c are
False in Box 5, cell b must be
True. In chain parlance a Strong link exists between cells marked c and d in Box 5.
OK, so cell d is
True. Well, since there can only be one 1 in Row 4, all the other 1's in that row must be False, including the 1 in cell e.
So, we have followed the chain via cells a-b-c-d-e and shown that if you assume that cell a is
False then cell e must be
False.
Well, what does that prove. Strictly speaking it proves nothing ! (Even though many sites will claim that it proves that the 1 in cell e must be
False and can be eliminated).
What is usually left out is the other half of the proof, although it's so obvious, its the reason that it usually doesn't get a mention.
Here is the other half : Assume the 1 in cell a is
True. Then since there can only be 1 True 1 in Column 9 all the other 1's in that column must be
False, including the 1 in cell e. A Weak link exists between the 1's in cells a and e.
Now we have gotten somewhere. Why is that ? Well, the 1 in cell a is either
True or
False. But it doesn't matter which is the case, in both cases the 1 in cell e is False, so we can eliminate the 1 in cell e without at this stage knowing whether the 1 in cell a is
True or
False.
Hopefully there was not too much Rocket Science in this explanation.
Leren