To further show the problem see the below: The first image is a BVH file movement imported to show what Layers could be used to do, it also is similar to what you get from Walk Designer. Notice each frame is keyed and lets say that frame 55 is to low and needs to be raised up.

Now in Poser 9 if I added a new Layer and made it an Add instead of Replace we get the following:

Now you could try to move each frame by hand a lot of work or as can be seen in the next image by moving just one point we can get a smooth slope that does not cause a jerking movement in the play out.

The next image is looking at the Base layer and you can see the RED line which is the current layer movement has remained the same. The Black Line is the actual movements that will be used in the animation. The next image is what happens when you convert the Mode from Add to Replace. In this image I have moved the First and Second Key Frames on the Adjustment Layer to change the overall shape. A Problem in all versions of Poser is the Last Frame in a Layer CANNOT be adjusted from Zero.

Now I will show what happens with Poser 11. We have the same BVH file:

And the Next Image shows what happens when you try to add an additional Layer to do the nice simple adjustment as was shown above in Poser 9:

The RED line sort of shows a Zero Addition to the overall influence to the Black Line, however Even though the 3 Key Frames on the RED line have a internal Value of Zero (if you were to look at the values in a saved scene) you would never know that by looking at the chart.
The next image shows a Serious BUG in 10 and 11 in that if you TRY to move the center point (any point really) it will jump to another Undefined value. Note instead of making the downward curve a little less sharp it now is inverted. And in the next image In poser 11 if you try you move the center point Notice it does not change the shape of the BLACK line.

As a last comment: In Poser 6-9 if I move the Adjustment Layer back and forth it will Simply do as the Poser Manual says the Add or Replace Modes work. However in Poser 10-11 It will Destroy the movements of the Base Layer especially as you have to switch back and forth between ADD and REPLACE Mode because Not all movement options work in Layers also won't work if more then 1 additional layer is added.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13