Sudoku Tutorial
Weak Links
When a candidate appears three of more times in a house, any pair of those candidates
share a WEAK LINK.  If one exists, it eliminates the other but it is possible that neither exists.

Any pair of 9's in this row is weakly linked.

Why do we care about weak links?  That's where advanced solving techniques like
X-wings can in handy.  If you can prove that a pair of 9's above is strongly linked, you can
eliminate the remaining 9.  (click here to see an X-wing example)

Weak links can also refer to different candidates in the same cell.

In the yellow cell, any two candidates in that cell are weakly linked.  They will eliminate
each other but possibly neither exists.