Phase Shift

Phase Shift makes the Imp invulnerable to all hostile and beneficial abilities, with the exception of Soul Link and Health Funnel.

Tips and Tactics
This spell is invaluable for those who rely on the additional health granted by the Imp's Blood Pact Aura, as it prevents the Imp from inadvertently being attacked or AoE'd: the Imp dies very easily, and the sudden loss of the Blood Pact Buff in the thick of combat can be most unpleasant.

If the Imp attacks another creature, Phase Shift is automatically cancelled; furthermore, it has a ten-second cooldown and cannot be cast while in combat. If you wish the Imp to stay phased, set it to Passive mode.

The Imp will still take damage from environments that cause damage (such as lava) while under phase shift. Whether this is a bug or intended by blizzard is still unknown.

Phase Shift can be set to auto-cast; when this is turned on, the Imp will automatically go into Phase Shift at the first opportunity when combat is over.

Phase Shift does not prevent the use of Dark Pact; in fact the two make a very beneficial combination, ensuring that the Imp's master always has a ready supply of mana.

Phase Shift prevents raid buffing to your Imp before fights. You can and should turn it off during buff times. The increased damage and survivability your Imp gets from buffs like Blessing of Kings or Mark of the Wild does make a difference.