You can tweak your render pass nodes, for example, via the Alpha value in the Layered Texture node, and visualize the changes interactively through IPR. 
The following figure illustrates the IPR results when the Alpha value of the Specular pass is set to 0.2, 1, and 3.
Note: When performing IPR, if you have a render target specified, IPR does not re-render the entire image, but only reflects updates to the way your passes are re-combined (for example, in this case, via the Layered Texture). The 
option in the Render View toolbar does not work with IPR. See Render View toolbar for more information.
option in the Render View toolbar does not work with IPR. See Render View toolbar for more information.