Lead lead dating
With 238U the Pb/U ratio grows much more slowly with age, but the idea is the same.
Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge.
But now imagine that some geologic event disturbs things to make the lead escape.
That would take the zircons on a straight line back to zero on the concordia diagram.
The straight line takes the zircons off the concordia. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched.
The results from these zircons therefore plot along that straight line, establishing what is called a discordia. If a 1500-million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the disturbance.