KILLING MACHINE: Your auto-attacks have a chance to trigger Killing Machine, which will make the next Obliterate or Frost Strike you use a guaranteed critical strike. You'll know when it's triggered, because your Obliterate and Frost Strike abilities will light up and two bars of ice will appear to the sides of your character. Obliterate will always hit harder than Frost Strike in every non-contrived situation, so you should always use KM procs on Obliterate if possible. Now, it's not always possible, and if Killing Machine appears in the split second before you hit Frost Strike….welcome to the world of Frost DPS. :P If KM procs when Obliterate is available or will become available in the next couple of seconds, it's worth waiting for it, otherwise just use the proc on Frost Strike.