from an idea of giorgio1957
in all the 9 sectors the position of a same digit CAN NOT be the same
that is: if in r1c1 we have 1, 1 cannot be present in r4c4, r4c7, r7c4, r7c7 (and, naturally, in r1c4, r1c7, r4c1, r7c1)
in our example ; in r3c2 we have 5, 5 can not be present in 56c5,r6c8,r9c5,r9c8