The Paladin sacrifices their life to protect a target party member. Enemies will stop attacking that party member and that party member will be completely protected from harmful attacks for 3 minutes. The targeted party member cannot take any action during this time. Interestingly, Mass Dispel cast by an enemy will not remove it. This requires a Symbol of Divinity as a reagent.
If you are a resurrecting class and the term "DI" is being thrown around, be prepared by getting as far away from any locations the monsters may return to as possible. If you get DI cast on you, the last place you want to be is in the middle of a monster camp when it wears off.
If you see that another party member is affected by Divine Intervention, attempt to die in a convenient location as fast as possible. If Divine Intervention fades while you are still in combat, there is a chance that the party member will die as well.
Divine Intervention lasts for 3 minutes, which should be enough for the rest of the group to die in a suitable location. Should the group die and the monsters reset before the full 3 minutes, the party member can cancel Divine Intervention by right-clicking on its buff icon.
Divine Intervention deaths count as suicides, saving the Paladin from taking durability damage.
When selecting a target to cast Divine Intervention on, the first choices should be another Paladin, a Priest, a Shaman, or a Druid. As a last resort, it may be cast on someone with Goblin Jumper Cables XL. Failing that, casting it on absolutely anyone will at least save two people from having repair bills.
If you have time, try to give a warning to the target of the spell to move to a suitably safe position. If either of you are going to die imminently, then you should cast the spell as quickly as possible and hope for the best.
Even if there are no targets who can resurrect still alive, it is still worth casting Divine Intervention to save yourself a repair bill (counts as a suicide, as mentioned above).
The target must manually cancel (right click) the buff in order to act before the timer is up. Some players will have no idea what is going on and be very confused.
Patch 3.3.0 (08-Dec-2009): This ability now also removes Exhaustion or Sated from a target if the recipient is out of combat when the effect ends. In addition, the cooldown on this ability has been reduced from 20 minutes to 10 minutes. Cannot be used in Arenas.
Patch 1.6.0 (12-Jul-2005): Fixed a bug where the Paladin did not always die when using the Divine Intervention spell.
Patch 1.3.0 (07-Mar-2005): Now castable on all raid targets.