The normal tool allows you to select layers by clicking on them in the canvas window. Multiple layers can be selected by holding the Control key while clicking.
Once a layer is selected, its ducks are displayed. The normal tool can select a duck by clicking on it. This will cause the duck to appear brighter. Again, multiple ducks can be selected by means of the Control key. Also, dragging a rectangle around a bunch of ducks will select them all, and Control-A can be used to select all the ducks in all selected layers.
Once some ducks are selected, the normal tool can be used to drag them around. This will change the values of the underlying parameters.