Sometimes two historical records incidentally touch on the same point in a manner that would be very unlikely if one of them were copied from the other or if both were copied from a common source. For example, one account of an event may leave out a bit of information, leaving some natural question unanswered, while a different account indirectly supplies the missing detail and, in so doing, answers that question. When this happens, the best explanation is that both records are grounded in the actual historical event; that is why the two bits fit together so well.
Forgers do not want to leave loose ends like this that might raise awkward questions; they take care to tie everything together neatly. But these are just the sort of things we would expect to find in authentic records of the same real event told by different people who knew what they were talking about.
He then goes on to give some key examples from the gospels: