Feint isn't free
Our old Glyph of Feint lowered the energy cost to zero, making Feint free to cast. Free is easy. We just used Feint on cooldown, or whenever we anticipating a burst of incoming damage. All that we needed was a free GCD, and rogues tend to have free GCDs in spades. We couldn't keep Feint up constantly, but that was never an issue. We used Feint with reckless abandon, as long as we were in melee range.
The new Feint has no cooldown. The new Feint can be used without a target and at any distance. The new Glyph of Feint extends Feint's duration. The new talent Elusiveness gives Feint superpowers. All of these changes make Feint great, but they come at a price. Feint costs 20 energy per use now, and that's means that we are choosing between damage and survivability every time we press it.
What 20 energy looks like
If we look at it from a simple viewpoint, 20 energy is about half of a Sinister Strike or about a third of a Mutilate. If we factor in the average damage of each of these abilities and add in the value of the partial combo point, we find that 20 energy is worth about 20,000 damage for a well-geared rogue. Considering that we're dealing tens of millions of damage per encounter, 20k doesn't seem like much. Seems like the choice is easier than we thought.
Using Feint once only cuts up to 66 DPS on a typical 5-minute encounter, which is a very tiny fraction of our six-digit DPS values. We could pop Feint every 7 seconds if we wanted to, and we'd lose less than 2% of our outgoing damage. Feint's cost is mostly symbolic. We should be using it whenever we can significantly reduce our incoming damage. There are very few situations where 66 DPS is worth taking unnecessary damage.
In fact, because of how minor the DPS loss is, Feint is safe to use to almost any time we're taking damage. Every time we Feint, we're putting mana back into our healers' pools. We don't need to just save Feint for the big explosions, but also to help with harmful DoTs or standard AoE and cleave damage. We shouldn't be holding back from Feinting just because we don't want to lose the energy.
GCDs aren't always free
Typically, rogues have plenty of spare GCDs to use Feint. However, if you're in the middle of an Adrenaline Rush or a Vendetta, you want to make every GCD count to maximize the cooldown. If you're capping your energy or missing a Mutilate in your Shadow Blades window, then you're losing way more than 66 DPS. We want to try to time our Feints so that we're not letting Rupture or Slice and Dice drop as we activate it. Unless you're going to die or take massive damage, your rotation should take precedence over using Feint.
The reverse case is also true. Because Feint costs energy and we often use it to counteract certain abilities, we occasionally need to pool energy before an attack comes in to ensure we have enough to get Feint off. There's nothing more annoying than spamming your Feint button, hoping you get to 20 energy before the boss finishes his big nova cast.
Feint's not our only tool
Evasion is often better than Feint at reducing incoming physical damage, and Cloak of Shadows is always better than Feint at reducing incoming magical damage. Both Evasion and Cloak are free to cast, and we should be working them into our defensive arsenal as often as possible. Feint is obviously going to be available at any time since it doesn't have a cooldown, but we shouldn't always be thinking of Feint first if there's another cooldown that's better suited for the situation.
Sneak in every Wednesday for our patch 5.2 guide, a deep-dive into the world of assassination and combat rogue AoE rotations -- and of course, all the basics in our guide to a raid-ready rogue.