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. When click over a layer you must consider to click over the visible part of the layer if it is a primitive like layer (circle, rectangle, region, outline, etc). If the layer is a "no edge layer" (gradient for example) you cannot click on the layers below although you have clicked in the trapsparent area of the layer. This allow to select only current visible layers. Consider also that some layers have not defined yet a way to click over them to be selected (Text Layer for example). Also if the layer is completely covered by other layer you cannot select it by a direct click in the canvas and you have to select it by the Layer List. (It could be a cool feature request though).
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. If you press CTRL meanwhile drag the selection rectangle then you toggle the selection of the ducks that are inside the rectangle.
Once some ducks are selected, the normal tool can be used to drag them around, this will change the values of the underlying parameters asociated to those ducks. You need to drag the selected ducks by clicking and dragging over one of them, otherwise if you click apart you would discard the current ducks selection.
If you press SHIFT key meanwhile you're dragging you can restrict the displacement to the X or Y coordinates of your displacement.
If you drag a tangent duck (or use a tangent duck to drag some selected ducks you have to press SHIFT after you have started to drag. If you press SHIFT before you start to drag, then it is interpreted that you want to split the tangents into individual left and right tangents and you (maybe) obtain something not desired.