As it is right now, Rune of Razorice causes 2% extra weapon damage as Frost damage.
The exploit is, this 2% extra weapon damage doesn't do that much damage, but the damage that it does do has a chance to proc Killing Machine, because the 2% damage is counted as another swing from the death knight. Any death knight specced far enough down into frost would know that killing machine is beast, whether you're dual-wielding or being an awesome shadowfrost spec, and doubling the proc rate is insanse.
Rune of Razorice - Spell - World of Warcraft
Killing Machine - Spell - World of Warcraft
Found over at Arena Junkies forums. Have fun while it lasts.
tested&confirmed.