This technique is my adaptation of work shown by Erik Dalton. I highly recommend his DVDs and home study courses which can be found at www.erikdalton.com.
I mention that I am using my knuckles to do the work, but it is actually the middle phalanges of my fingers. That works better than knuckles because the wrist can be kept straight, whereas it is flexed when you use knuckles.
It is important also to work on the front of the body, working pectoralis major, pectoralis minor and other muscles that are pulling the shoulders forward and contributing to the dowager's hump. Front line work from the work of Tom Myers (www.anatomytrains.com) is excellent for this.
A variation is to have your client tuck their chin, then lift their head their head up while keeping the chin tucked. This isolates the extension to the lower cervicals and upper thoracics rather than the upper cervicals.
Pectoralis Minor - Soft Tissue Release