What is the relevance of Pratityasamutpada in Nagarjunas system?

In his philosophy, Nagarjuna uses the Pratityasamutpada to prove, that everything is empty and without reality. In the end, however he proves not only the emptiness of that very Pratityasamutpada but he also somehow leaves the law (of dependent arising) intact and working. Does anyone understand his point, especially in what concerns Pratityasamutpada?