You'll need to perform an autopsy of a Sectoid Commander to unlock the Neural Feedback Gene Mod. Get a Sectoid Commander or an Ethereal to less than 3 health and let them psionically attack your soldier equipped with the Neural Feedback Mod.
Tips from PromoteViolence:
The lowest Neural Feedback damage I ever did to an enemy was 8, and I've seen 13 done to an Ethereal on Normal difficulty. I believe higher Will raises damage, and maybe a Will vs Will type situation is taken into account. When I did 13 damage, my soldier only had 138 will (No Psi Armor or Psi Inspiration, just a Mind Shield and medal bonuses). I would recommend only giving this ability to higher Will troops.