Example: You are armed with a longsword. You are facing a monster with DR 5/bludgeoning. Clearly a weapon that deals slashing damage is not the best choice against such a monster. But there is hope.
In historical swordsmanship there is a technique called the Mordhau, the murder strike. It involves flipping your sword around and holding it in such a way that won't cut your fingers off when you hit someone with your pommel or crossguard, effectively turning your slashing weapon into a bludgeoning weapon. But would such an attack make your weapon an improvised weapon, and hence suffer the -4 penalty on an attack roll, or will it not suffer this penalty and allow you to attack normally?
I would prefer a RAW answer if it exists.
Best Answer
It would be an improvised weapon.
A special attack such as that would exist in a feat, whether it be core, supplemental, or third party (or a class feature). An example of a feat would be:
What you have done, is essentially turned your sword into a metallic club, and would deal damage as a club of its size as an improvised weapon. You are indeed, improvising, in using your sword in such a manner. If a sword was literally designed for such an attack, it would have a pommel on the end of the blade.
DM Ruling
Speak with your DM. He may be agree to lessen the penalty by half or so depending on the role-playing aspects, weapon focus style feats, and etc. Paizo also has Improvised Weapon Feats that can remove the penalty altogether no matter what you want to use as what kind of weapon. He may allow those feats into his campaign.
Key Point
The sword was not designed for such an attack. Just because the wielder improvises, adapts, and overcomes is merely a testament to his knowledge as a martial artist. Doing the best with what you have and making more with less is the improvisational and technological mantras that helped make us who and what we are.