When you gain mana from Drain Mana or Life Tap spells, your pet gains 100% of the mana you gain.
Mana Feed is a Warlock talent in the Demonology tree that causes Life Tap and Drain Mana to also restore a percentage of the gained mana to the Warlock's pet.
% Mana Gain Transfer
Mana Feed is especially useful for grinding, where the paltry mana totals of a Voidwalker or Felguard might come up a bit short, or to recharge the waning mana of your Felguard or Imp during long battles.
Some people are apt not to use Suffering because of the large mana cost, but this talent compensates for that and allows for the Voidwalker to be a much stronger tank. That said, if you have a lot of +dmg gear, you may not need to buy in all the way, since pet mana totals are quite small.
Also, it is possible to talent for both Dark Pact and Mana Feed, which combined with effects like Siphon Life can turn your pet into an almost inexhaustible mana battery.
Patch 3.1.0 (14-Apr-2009): This talent is now a 1-point talent, down from 3-points. Now is the 21-point talent in Demonology. Now grants 100% mana return to your pet, up from 33/66/100%.