With a throwing weapon (fire bottle, dynamite, or knife) in hand, enter Dead Eye mode, and try to imagine the normal targeting reticule on the screen.

The target will not appear, but it works the same.

If you rapidly tap RB, it will be easier. When you release the object (automatic after locking on), the projectile will home in on the target.

This is really helpful when people are speeding away on horses/carts.

The projectile will keep flying until it hits the target.

Try this with a stick of dynamite on birds; the effect is hilarious.