It requires that once the plaintiff has put forward prima facie evidence, which means enough evidence on its face, a court could draw an inference on a balance of probabilities that the harm had been caused by that defendant. So essentially, in a tennis game the plaintiff has to serve first. They don't have to prove it 100%, but they have to get to the point of putting forward a good-faith balance of evidence that this has occurred.
On October 27th, 2010. See this statement in context.