After obtaining Launch by passing through Communications and into Pneumatics, you'll want to invest some ability points in the Launch skill tree. The first sub-upgrade that is offered will allow you to grab grenades and rockets thrown/launched by the enemy. There are enemies that will solely shoot at you from afar with a guided rocket, and then there are larger enemies with a grenade launcher later on; these enemies will frequently attack with these, so aim towards the projectile and use Launch. With some success, it will psychically attract the projectile and allow you to throw it back.