daj95376, in classical logic theory, if a or b, there are 3 possible cases:

1. a=true and b=false

2. a=false and b=true

3. a=true and b=true

So in this particular instance, it turns out to be case (3). So the or relationship is not violated.

Basically, we just need to prove at least one of r5c6=9 or r6c8=9 must be true. That they are in fact both true doesn't violate our conditions...

Of course, an immediate result of r5c6=9 or r6c8=9 is that they must both be true (from the strong links on r5 and b6). But it was Carcul's choice to use that particular or relationship from the start and he's in the position to explain it... I'm as puzzled as you on why he chose to take that particular path...