Not sure if this will help, but if I understand correctly, it goes like this:

Let's say a woman has only sons. The sons have her mitochondrial dna, and have children. The grandmother's mitochondrial DNA is left behind because, apparently, when the sperm hits the egg, it destroys the mtDNA it carried with it. And, because nature is devious, that doesn't always happen completely. But usually, then the grandchildren will have only their mothers' mitochondrial DNA. However, any of the granddaughters will have their paternal grandmother's X chromosome.

And for further fun, because our chromosomes line up and split apart without regard to whether they came from the mother or the father in the first place, a grandchild might easily have very little from one or another of their grandparents. Like the time the father had passed away, and his mother was sure one of the grandchildren wasn't his child. The DNA test didn't detect enough relation to the grandmother, so they tested against the siblings, which worked because they were full siblings, not half.