here is something that may explain it a little clearer... i do hope this pleases Naugladur... :rolleyes
How was Eowyn able to kill the Witch-king?
This is a biggie that people come up with different theories on. First is that she was able to do it because she was a woman, not a man, since the Witch-king is obviously convinced that no man can kill him (this is made extra confusing with her response "I am no man"). The Witch-king's source of that information was a prediction by an elf called Glorfindel, who said that he would not be killed by a man (which of course, he wasn't). So it wasn't that she was able to kill him in a way in which a man would have been unable -- it's just that she fulfilled a prophecy.
There's also contention about the role of Merry in this whole thing. Merry (we think, although I've heard different opinions on this as well) stabbed the Witch-king in the leg with a Westernesse blade (the one Aragorn gave him on Weathertop in the first film), which was forged specifically to fight against the Witch-king (I believe before he actually became the Witch-king). Tolkien doesn't specify, but the theory is that this is a special blade that somehow rends the essence, or the fabric, if you will (not the clothing, but rather the force that holds him together) of the Witch-king, which is what makes Eowyn ultimately able to kill him with her plain old Rohirric blade. There are multiple camps on this, as neither of them are men: some people think that Merry (a hobbit) was actually the person who killed the Witch-king. Others think that it was all Eowyn (a woman). I personally am of the thought that it was both, and one could not have done it without the other. In my opinion, this is the theory that is the most supported by the information from Tolkien.
It should be noted that this is a contentious point, since there have been people who have said that Merry was not actually given a Westernesse blade on Weathertop in the film, and also that the contingencies that allowed Merry to get his blade back after being kidnapped and disarmed by the orcs in the book was not present in the film, so that Merry was armed only with a plain old Rohirric blade given to him by Eowyn. It's possible that, to simplify, the director decided to do away with the Westernesse angle altogether. Merry distracted him, and Eowyn dealt the death blow