How could Madara free himself from the Impure World Resurrection technique, while Tobirama can't? In Chapter 591, we see Madara Uchiha free himself from the influence of Kabuto's Impure World Resurrection/Reincarnation technique (Edo Tensei), saying that he was able to do this because he knew about the technique's seals.



However, in Chapter 620, we see the inventor of the Impure World Resurrection technique, the second Hokage, Tobirama Senju, wanting to be free of the technique, but was stopped by Orochimaru. 



How was it that Tobirama was not able to free himself from the Impure World Resurrection technique, despite being its inventor and knowing all about the technique and its seals, but Madara can?



Credits: Linked images are from Mangastream's scanlations.
When Madara frees himself of the Edo Tensei, he performs the seal. But in the case of Tobirama, you can see that in the first attempt he only tries to crush the wall. Also in the next page you can read this:

Hashirama: You increased the restraining power using my cells.
Tobirama: Now that I sense better, his body is almost made of my brother's cells.

So Madara was able to free himself because no one restrained him before he performed the seal; in the case of Tobirama, he tried to destroy the wall first and Orochimaru was able to restrain him with the increased restraining power he has.