Your AI soldier buddies have rather suicidal tendencies and will occasionally get downed. You can tell they are in this state when a small green + symbol appears next to them. Boost over to them and interact with them (x by default) to administer a first aid shot. Be aware that you aren't immune to damage during the animation and that if you take to long the soldier can bleed out. You will be rewarded with a weapon drop and the achievement for your efforts.