supplementary scratch
A.k.a. "courtesy scratch"; it involves your helpfully scratching someone's back in a spot where he has difficulty reaching himself.
Giving someone a supplementary scratch is indeed a very nice gesture, but da prob is dat you cannot know where it itches on your friend's back, and so you may hafta do a good bit of "up/down-left/right" trial-and-error scratching till you finally hit da right spot.