Burden of proof is a very straightforward concept, really.
Generally, when you go into a legal action, the burden of proof rests with the complainant. With whistle-blowing legislation, it shifts the burden. In this instance, it would be to the government, which would have to prove that it did not retaliate because the person blew the whistle. It changes everything. It gives the whistle-blower a winning chance. Without that shift, they won't prevail.