The idea behind the user drilling the hole to bolt the driver on, is you can set the depth to the tool so the alignment bar end is inside the coupler and seats the bearing before hitting the coupler with the shoulder on the alignment bar. If you set it with the driver part too far in, you can drive the end of the bar into the side of the coupler possibly damaging it while you pound with a hammer. If you set with the driver too far out then the shoulder on the bar will hit the coupler before the gimbal bearing is all the way in.
That's the theory anyway. Problem is, most people get their bar and drill it after removing the gimbal bearing. Makes it tough to know right where to drill the hole.