As stated, Void Shift causes the targets to swap their current health percentages. Since different players have different maximum health amounts, this can result in an overall loss or gain of health. If the priest and/or the target is below 25% health, Void Shift will increase the lower health percentage to 25%.
Void Shift can be used for the benefit of either party; the priest may sacrifice their health for the good of an ally, or steal another's health for themselves. Depending on the situation and the priest's role, either option has the potential to be either a wise or a selfish act.
The prudence of using Void Shift depends to a large extent on the subsequent survivability of the two targets, and the relative value of their survival. When the use of Void Shift prevents either party from dying, it can be considered to have been a wise act; when it sacrifices the life of one party in order to save the other, its prudence must be calculated accordingly.
Shadow priests can make use of Void Shift to great effect, either in sacrificing their own health to save a tank or ailing ally, or in stealing another's health when the priest's survival is of the greater benefit to the team (such as when carrying a flag or orb).
When resulting in a loss of health for the target, Void Shift has the potential to be extremely aggravating, especially if its use is not announced. Whenever possible, communicate with the target and inform them of your act; assuming the decision was a reasonable one, they should understand the necessity of its use. However, as with all such abilities, its use is at the discretion of the priest, and while all efforts should be made to avoid aggravation, when the situation demands it, the ability should be used without hesitation.
In the absence of any greater benefit to the team, Void Shift should not be used simply to extend the priest's life at the cost of an ally's; such behavior is anti-social and is likely to result in the priest being kicked from the group or refused support or healing in subsequent encounters. Of course, when the target is at little risk, this use may be an acceptable way of allowing both parties to survive the encounter; subsequently healing the target may go some way to making up for the theft of their health!
Void Shift's benefit depends directly upon the health of the participants when it is used. Generally speaking, the greater the contrast in the targets' health, the greater the potential usefulness. Targets with roughly equal health percentages have little or nothing to gain from using Void Shift, unless one of them is under 25% health.
Despite its healing potential, Void Shift is in the Shadow school. This can make it useful as an emergency life-saving option for priests who have been locked-out of the Holy school.
With a relatively long cooldown, Void Shift is arguably best saved for emergencies.
Patches and hotfixes
Patch 8.0.1(2018-07-17): Converted from a rank 46 honor talent to a PvP talent.
Patch 7.0.3(2016-07-19): Re-added as a Shadow honor talent. Cooldown reduced from 10 minutes to 4 minutes.
Patch 5.3.0(2013-05-21): Cooldown decreased to 5 minutes, down from 6 minutes for Discipline and Holy Priests. For Shadow Priests, cooldown has been increased to 10 minutes, up from 6 minutes, and the spell is no longer usable while in Arenas or Rated Battlegrounds.
Hotfix (2012-10-16):Void Shift can no longer be used on NPCs, including player pets.
Hotfix (2012-10-12):Void Shift can now only work on other players.