In "getting down low", "down" seems to give the image of going down, but why should the word "low" come next to "down"?
"Low" is used after "down" to indicate the level, to show that the body has been lowered in position closer to the ground. Without the use of this word, we would only know that the body has been lowered and not the depth of the action. To "get down low" usually indicates that someone has lowered themselves very close to the ground or floor, and is even possibly lying down. Ex: I think the robber has a gun. Get down low. Ex: To play limbo, you have to get down low.