Complete the "Dossier: Tali" mission from your second set of Dossier missions to unlock this.
During your battle with the Geth Colossus, refuse the help from Kal'Reeger (it will take a Paragon action). If any of your squad dies, go talk to him and refuse his help again. It will bring the squadmate(s) back to life without having to use your Unity/Medi-Gel.