I didn't notice or need this until I started a play through on Insanity difficulty, but to use a companion's power it only requires Shepard to have line of sight to the enemy. If the companion does have line of sight they will pop out of cover and deploy their power, which exposes them to enemy fire and gives their target time to take cover, possibly thwarting the attack. No surprises so far, but here's the trick: if Shepard has line of sight to the enemy but the companion doesn't, neither Shepard nor the companion will break cover and the power affects the enemy instantaneously. If a companion has strong powers and mediocre weapons, station them well back from the action where they won't take fire and can't see anything. For example, station Miranda in another room while you fight so her Warp and Overload will instantly and automatically hit any enemy that Shepard can target, you won't have to waste any medi-gel to keep her on her feet, and you probably won't miss her ineffectual SMG fire. Vorcha with rocket launchers pummeling you from cover on a ledge you can't reach? Station Miranda and Jack in the next room, wait until a Vorcha stands up to shoot, use Miranda's "insta-warp" to take down their armor and Jack's "insta-pull" to yank them off the ledge, and Shepard only needs to break cover to finish them off if the fall doesn't kill them.