
The four
yellow cells have the potential to form a deadly rectangle.
Look at the yellow cells in column 4.
The trick here is to look for a strongly linked 4 or 8 in column 4.
The rule is this: "Of candidates X and Y, if candidate X is strongly linked, candidate Y can be eliminated"
In this case, the 8's in column 4 are not strongly linked (there are
four of them). However the 4's are strongly linked (there are
only two of them). So if either of the yellow cells in column 4
is an 8, the other yellow cell is forced to become a 4. This
would complete the deadly rectangle. So both 8's can be
eliminated.

