For those fortunate Rogues, Windwalker Monks, Feral Druids, and Enhancement Shaman, there is a trinket of great power known as Soul Capacitor.
In this topic, I will be detailing how the trinket works, when to use the macro for certain encounters, and which encounters you shouldn't bring it at all.
The Trinket Proc Itself
When it activates, all of your personal damage done is absorbed for 10 seconds. When cancelled or once the effect ends, you will cause an AoE explosion dealing 120.77-137.44% of said damage (based on item level) to everything within ten yards and it is split evenly among them. Only damage from the player him/herself counts toward this proc. Any damage from pets or summoned creatures is not absorbed during this effect.
Damage absorbed during a Soul Capacitor proc will count towards the effects of abilities such as Ancestral Guidance and Nature's Vigil. Note that the explosion damage itself will count for AG but not for NV.
Abilities/creatures ignored by Soul Capacitor
Force of Nature
Invoke Xuen, the White Tiger
Storm, Earth, and Fire (It will be better to use this ability than equipping Soul Capacitor on certain encounters. Use your best judgment.)
Shadow Reflection
All Greater/Primal Elementals
Searing and Magma Totem
Minions spawned by the Mirror of the Blademaster trinket
Legendary Ring Interactions
The trinket's explosion damage is increased by 25% during the ring's effect and counts toward the ring's explosion.
Absorbed damage during the trinket's proc does not count toward the ring's explosion.
Damage done by the ring's explosion during a trinket proc will be absorbed and count toward the trinket's explosion.
The ideal scenario is that Soul Capacitor procs while the legendary ring is active and explodes before the ring's effect is lost.
The Macro
If you are using Soul Capacitor on any fight, you are recommended to have a macro with the simple line "/cancelaura Spirit Shift". This will allow you to immediately trigger the trinket's explosion so that it properly hits an intended target, does not waste damage, or cause mechanical failures that can potentially wipe the raid.
The WeakAura
http://pastebin.com/HegyFDbS - This WeakAura will show you how much time you have until Soul Capacitor naturally explodes and for how much damage the explosion will do.
Individual Fight Recommendations
Bellow are some basic recommendations/thoughts for each fight in Hellfire Citadel in regards to the trinket and macro. There are some fights were Soul Capacitor should absolutely not be used and other trinkets appropriate for the situation should be taken instead.
Hellfire Assault
Have the cancelaura macro ready, should you need to move across the area to another mob. While focusing down Grute or Grand Corruptor U'rogg, Soul Capacitor procs will grant you a 10 second window to AoE without consequence and focus all of that absorbed AoE damage into the primary target. As a general rule, AoE is 100% safe while a Soul Capacitor proc is active, as long as you can ensure the explosion hits the right primary target(s) within 10 yards of yourself.
Iron Reaver
Cancelaura macro should be ready for Blitz and for exploding immediately to take out bombs in all phases.
Kormrok
Immediately on the pull and for all phase transitions, use the cancelaura macro as soon as you see the cooldown timer for Leap fall below 3 seconds. Holding a Soul Capacitor proc any longer than this will risk that the explosion will miss the boss entirely. Soul Capacitor procs during hands can be very helpful for dealing big damage to the boss, as the hands will almost always die earlier than the proc ending.
Hellfire High Council
Soul Capacitor's explosion can cause issues in taking down Gurtogg Bloodboil in a timely fashion. In regards to our present Mythic progression, Soul Capacitor proc damage split with Blademaster Jubel'thos can be the difference between 20% or 30% health lost from dealing with Bloodboil. Until we can safely kill Gurtogg before 3 stacks with all Agility users wielding Soul Capacitor, DO NOT USE!
Kilrogg Deadeye
Perfectly safe to use this trinket, provided you are not assigned for Visions of Death, especially on Mythic. Soul Capacitor procs during Visions of Death will make it much harder to obtain the 20 stacks, leading to much less overall DPS on the boss once you are expelled from the vision.
Gorefiend
Numerous opportunities exist where you can be screwed over during a Soul Capacitor proc, especially off the opening round of Shadows of Death. On Mythic especially, it is a 100% DO NOT USE EVER, as you are capable of killing Corrupted Souls with the explosions and forcing people out of the stomach early can inevitably cause wipes.
Shadow-Lord Iskar
Unless AoEing during the transition phases is a single-target gain, avoid doing us unless Soul Capacitor is active. Regardless of class, a Soul Capacitor explosion will be of much greater benefit from you AoEing in that scenario than pure single-target alone. This is also advantageous if weaker adds are dying prior to the explosion, giving you a much stronger split for damage between any left alive.
Fel Lord Zakuun
There are almost no scenarios where you will need to use the cancelaura macro. You should use the macro, only to cancel an explosion before running out with Corrupted Soul. Otherwise, Seeds of Destruction will not last long enough to necessitate you cancelling the aura.
Xhul'horac
While the giant add from Phase 1 or 2 is active, Soul Capacitor explosions will hit both mobs for enormous amounts of damage. Use the macro if a proc is active and you need to run out for Fel Surge.
Sorcrethar the Eternal
Have the macro ready for when a proc happens and a Sargerei Dominator is active. Waiting for the explosion only makes the dominator die less quickly and the explosion will split damage with an immune Sorcrethar.
Tyrant Velhari
Unless directed to cleave damage on any of the adds, it is not advantageous to do so, even during a Soul Capacitor proc. Have the macro ready during Phase 3, unless the Ancient Sovereign can be kept at least 10 yards away from the boss.
Mannoroth
The time required to move out of various abilities is minimal and should not impede the use of the trinket. All other targets in the fight are beneficial for you to be hitting with the explosion, when possible.
Archimonde
Before the first Soul Capacitor proc explodes off the pull, position yourself within melee range of the Doomfire Spirit but as far away from Archimonde as possible. This will ensure that the first explosion hits only the Doomfire Spirit and deals with it quickly. Watch for when the trinket procs and if possible, attempt to do the same with all other Doomfire Spirits for the rest of the fight. Otherwise, it is perfectly fine for explosions to hit both Archimonde and the Hellfire Deathcaller. The most ideal scenario is for Soul Capacitor to proc, but not explode until shortly after the Deathcaller dies, hitting Archimonde alone for the full damage. If you are assigned to a Nether Banish group, keep a close eye on the Shadowed Netherwalker's health and use the macro to cancel early, if you feel you can get out of the nether faster by doing so. For Infernal Doombringers in the final phase, always have the macro ready to cancel the proc if necessary before the Infernal dies. Having your Infernal die while the proc is going on looks really bad for you in logs and may ultimately waste damage if you cannot get to another Infernal or Archimonde in time for the explosion.