Double-clicking a Layer in the Timeline provides different results and is dependent on the type of layer you apply the double-click on. If the layer is a normal layer (not a nested composition) then the Layer Panel will be invoked and the layer will be displayed. If the layer you double-click is a nested composition, then the nested composition's original/source composition will be invoked. If the layer being double-clicked has its Collapsed Transformation or Continuously Rasterize switch enabled, then double-clicking on such a layer does nothing. This is the case since AE prevents such layers from appearing in the Layer Panel.