In my opinion it means that as the arrow is released, there is a "twang" sound that the deer hears just as the arrow begins flight. When the deer hears this, his natural reaction is to duck or lower his body (as we would do if startled by a loud noise). When this happens, the deer lowers his shoulders just as the arrow flies over his body.

The point is, had the deer not reacted to the sound of the arrow being released, the arrow would have flown true, hitting the vitals. Others may have a better explaination.....