As hunters, our main job in a raid or a dungeon run is to do as much eyebrow-searing DPS as we can, and do it without pulling aggro or standing in void zones. This is the primary difference between hunters and healers -- healers don't worry about any of these things. One of the secrets of end-game hunter DPS is to stack our cooldowns together, and to time those cooldowns with trinket procs. Over the course of a boss fight, this stacking can yield very real DPS gains. While every hunter spec benefits from this stacking, marksman benefits the most, being able to use our big cooldown, Rapid Fire, four times in a typical four- to five-minute fight. The key here is to stack abilities that combine multiplicatively, rather than additively.
One of the interesting benefits of the DPS gain of stacking cooldowns is that you could actually do more DPS than spreadsheets would indicate. Spreadsheets use averaging of all abilities over the course of a fight, rather than assuming that you're combining them intelligently. Join me after the cut as we take a look at how it works, when you should stack and when you should not.
Multiplicative versus additive
An additive combination, for the purposes of this discussion, is two abilities that combine to do the same total DPS as when they're used separately. So if abilities A and B give you 10 DPS each and they're additive, using them at the same time gives you 20 DPS. Using them separately, one after the other, also gives you a total of 20 DPS. Attack power combines additively with more attack power. So you could stack your Furious Howl with your Mark of Supremacy, but it's no more advantageous than using them separately.
A multiplicative combination is two abilities that combine to do more total damage than they would separately. So abilities A and B are 10 DPS each, but by using them at the same time they give you 25 DPS. Generally you'll get this multiplicative advantage any time you're combining two different kinds of abilities: haste with attack power, or crit with haste, attack power with crit, or agility with ... well, agility with anything, including agility, is a multiplicative advantage.
A good example is your wolf's Furious Howl and Rapid Fire. Furious Howl adds 320 attack power, while Rapid Fire adds 40% haste. By using both together, the DPS boost is greater than if you used them separately (because now you're not only getting extra shots from Rapid Fire, but those extra shots also have an additional 320 attack power as well as all the other shots that would normally benefit from the attack power).
The trinket factorMost hunters will either have the Darkmoon Card: Greatness or Death's Verdict trinkets. Each of these has a 45-second internal cooldown and procs for a whole bunch of agility. That agility gives you attack power and increases your crit chance. So if you use Furious Howl with your trinket proc, the 320 attack power from Furious Howl will crit more. If you use the proc with Rapid Fire, those extra shots will do more damage and crit more often.The more you stack together, the bigger the multiplicative advantage.
However, it would not do you any good to stack your Whispering Fanged Skull proc along with your Furious Howl, since they both give additional attack power they stack additively. It won't hurt at all, mind you; it just won't be any better than using them separately. However, if you toss in a Rapid Fire with either or both of them, now you're seeing the multiplicative advantage.
There are two ways to track your trinket procs. The first is just to configure your Power Auras (or other notification addon) to let you know when your trinket procs. Alternately, you can get an addon like SexyCooldowns that will track the internal cooldown of your trinket. Both are viable methods of stacking cooldowns with trinket procs. Personally I use Power Auras to let me know when my trinkets proc, and I know when my Death's Verdict is about to proc because it's right after Furious Howl comes off cooldown.
When not to stack them
In general, you always want to stack as many cooldowns together as possible. If you've got Rapid Fire going, pile everything else on it that you can! This is particularly easy at the beginning of a fight, when any proc trinkets you have will all be going off at once. Death's Verdict, Greatness, Deathbringer's Will, Whispering Fanged Skull -- all of them will trigger in the first few seconds of a fight. Toss on Furious Howl, Rapid Fire and Call of the Wild, and you have a potent multiplicative advantage.
However, later on in the fight, these cooldowns will all separate from each other, and you will not have the luxury of stacking them all together. One of the mistakes I often see is people's holding off on their cooldowns, waiting for that magic moment when everything lines up -- not just trinket procs, but the tier 10 set bonus proc as well (which is not predictable).
While it can be worthwhile to wait a bit on cooldowns, you absolutely never want to put yourself in a situation where you use them fewer times than possible because you're waiting for some magical combination. This is a particularly common mistake with MM hunters who are holding off on Rapid Fires. In a five-minute fight, a MM hunter will get four Rapid Fires. It is easy to combine these with trinket procs (trinkets with 45-second internal cooldowns, like DV or Greatness). You end up sitting on your Rapid Fire for a chunk of time, but you still get all four uses.
However if the fight is shorter, less than four minutes long, you no longer have the luxury of waiting on those trinket procs. If you wait, you'll only get three or even two Rapid Fires in the course of the fight. You are better off using Rapid Fire occasionally with nothing stacked on it than using fewer Rapid Fires.
In general, I blow everything just after engagement. I time my Rapid Fires based on the duration of the fight -- longer fights I can stack them with trinkets, shorter fights I don't. I also manually cast Furious Howl with every trinket proc -- the cooldowns line up very nicely -- and of course you can cast it while doing your normal rotation. Unfortunately, Deathbringer's Will has an oddly long internal cooldown and doesn't line up nicely with other cooldowns on most fights, though of course you can always rely on it at the beginning.
But I never worry about set bonus or other totally random procs -- if they happen, that's great, but I'm not waiting on them. If I know in advance when Heroism/Bloodlust will be, I'll try to plan around it, but again, I'm not going to waste time waiting on them.
Like everything in life, it's just like beer
Stacking your cooldowns will net you a nice boost to your DPS, but like so many end-game techniques to increase DPS, the difference isn't huge. It's similar to epic gems vs. rare, or the best enchants vs. the cheap ones -- the increase is small, but it all adds up to make the difference between a great hunter and an average one. It's like after bar closing when you've finished your last beer; you can still wring out the bar rags into your mug rather than let it go to waste (the dwarven hunters know what I'm talking about). At the end of the day, why leave any DPS on the table?
You want to be a hunter, eh? You start with science, then you add some Dwarven Stout and round it off some elf-bashing. The end result is massive DPS. Scattered Shots is the WoW.com column dedicated to helping you learn everything it takes to be a hunter. See the Scattered Shots Resource Guide for a full listing of vital and entertaining hunter guides, including how to improve your heroic DPS, understand the impact of skill vs. gear, get started with Beast Mastery 101 and Marksman 101 and even solo bosses with some extreme soloing.