If its long in the back it needs to be flexible because it has more body to move. A shorter backed dog can turn a ninety degree angle in a pipe quicker and easier than a long back. A shorter back can manouvre quicker and easier in a tight space ie oven, because its feet are in contact with the ground more often. Its the diffrence between a mini and a coach.. Big lorries need to be articulated to manovre round a tight corner but a short vehicle does not, thats why I say if a dog has a long back it needs to be flexible where a shorter backed one doesnt.. I dont think extremes of either are good