Preview Mode

There are two preview modes, one above the channel list and the other one is above the layer list. Channel preview mode lets you see the entire output of the selected channel. If you click the normal channel several times, it will cycle between normal and it's bump value.

Layer preview mode lets you see only one selected layer. By default, you'll see a the selected channel value of the selected layer. But you can also change settings on the right side of the preview mode button to change it to only show the layer's alpha value or a specific mask.