The occlusion method creates a mask from the depths of the objects on the screen, blurs it away from the light, and uses it to mask fog and atmosphere. This acts more like real life light shafts in that the shadowing of the fog is what creates the light shafts. Note that this also means that your light shafts can only be as strong as your fog / atmosphere. This is intended to be used in games trying for a more realistic look.
The sun directional light can be slightly off the screen and still produce occlusion light shafts, however, they currently fade out as you approach a 90 degree angle from the sun.