Modifying a Duplicate Layer/Layer Mask

[Ofnuts]

When you add a layer mask, all changes you do next apply to the mask and not to the layer (until you explicitly reset the layer th0umbnail). Changes in the layer mask only change the opacity of the layer. Since the layer you work on is a copy of the layer under it, when you make part of it transparent you see the layer below but since it is identical you don't see anything changing.