You have X% chance to heal your pet for Y% of the spell damage caused by you
Fel Synergy is a warlocktalent in the first tier of the Demonology tree. The warlock's spells have a chance to heal his minion for a percentage of the damage done.
Chance to heal
Since this talent is based off of spell damage you cause, damage from wands will proc the heal.
Patch 3.1.1 (2009-04-21): Tooltip has been updated to indicate 15% of spell damage 50% of the time at rank 1
Patch 3.1.0 (14-Apr-2009): Has been moved to tier 1. It no longer increases intellect, stamina and damage of the summoned demon. Healing done is increased in the current test build - 50% of spell damage 50% of the time at rank 1, 30% of spell damage 100% of the time at rank 2.
/Patch 3.0.2 (14-Oct-2008): Fel Synergy has been added to the game.