Sometimes losing the small battle will let you win the war.
To be honest I cannot remember the exact situation that this rule refers to. However the principle is sound. There have been many times that I have conceded on a small issue so that I could win a larger one. For example (and I think this was the basis for this rule) I wanted a new TV for the bedroom. My wife wanted one too but was not sure that we should spend a lot on a new one she thought we should borrow one from her family. I agreed and said we should not spend a lot of money on a new tv for the bedroom, but instead we should move our living room TV into the the bedroom and get a new bigger living room TV. She said no we should get a smaller TV for the bedroom but we could get it new. I agreed and we got a new TV. My wife was happy, I was happy and it was all because I gave up on the little battle to win the big one.