**Terminology**
Rod Hagglund called the pattern involved a 'hinge'. `Havard` calls it
an 'empty rectangle'.

Starting with the given 17-clue puzzle, one easily arrives at the second diagram above.

Now where should the digit 1 be in the middle bottom box? It is in one of the four yellow squares, and that means that fields a and c cannot both be 1 - that would cover all yellow squares and not leave a place for 1 in the middle bottom box. But there is a 1 in the second row, and that 1 is either in field b or in field c (because the other two empty cells in that row already see a 1 in their column). We see that if a is 1, then b is not, and c is, impossible. So, a is not 1. The rest is straightforward.

**Terminology**
We called two positions such that at least one must contain a given digit (like b and c
for the digit 1) a *pair* for that digit. People also use terms like 'strong pair'
or 'conjugate pair'. Then two positions form a 'weak pair' if at most one contains
a given digit (like a and b for the digit 1).