Basically the left trigger puts the ball EXACTLY where they are at when you pass it (if a bullet pass). Adding in a direction with a LT bullet pass gives your receiver a VERY slight lead, no where near as drastic as a normal bullet pass. A normal bullet pass leads the receiver's route, where as a LT bullet pass leads the receiver from where they are at exactly when you throw. So if a receiver is in between coverages (from the left and right) a LT bullet pass will hit them right where they are at, resulting in a completion. A normal bullet pass would lead right into one of the two coverages, resulting in a pick or knock down.
Hope you can understand what I mean.