Killer pairs are one of the regularly used techniques for solving killer sudokus. They are well explained in many solving guides including the ones linked from my post Basic Techniques and Cage Combinations.
This one is a very unusual killer pair from Afmob's walkthrough for A177 here.
1. R789+N5 !
...
d) Innies R789 = 15(3) = [168/276] since R7C59 = (678); 6 locked for R7
e) ! 7(2) <> {16} since it's a Killer pair of Innies R789 (R7C1 = 1 xor R7C9 = 6)
I hope that Afmob won't mind me explaining this in a bit more detail.
Innies R789: R7C159 = 15(3) = [168/276]
If R7C159 = [168] => no 1,6 in R7C8 => no 1,6 in 7(2) @ N9
If R7C159 = [276] => R7C9 = 6 => no 1,6 in 7(2) @ N9
An alternative way to achieve this breakthrough is to use an unusual CCC.
Thanks Ed for providing the diagram!