An effect is applied to a selection as soon as you click its preview in the Designer Effects workpane. This means that you can immediately see how the effect changes your selection. If you want to layer one effect on top of another effect, however, you must lock the first effect. Then, when you select another effect from the gallery, it will be applied on top of the previous effect.
If you want to experiment with different combinations of effects, you can layer specific effects to create unique pictures.
Tip If you want to modify only part of the object, you can use options on the Cut Out workpane to select the areas you want to change. Ways to select an area