The GURPS roleplaying supplement New Sun, based on the Book of the New Sun novels by Gene Wolfe, describes a particular sword as having a hollow channel in its core containing mercury. According to the material, this causes the blade's weight to "shift backward when raised and rush forward when swung" (words in quotations are the entry's exact phrasing).

I assume this is supposed to give forward strikes more force, but I honestly wonder- would such a design contribute to or take away from a sword's effectiveness?