Digital Light & Color

From the built-in help :

Shift-Click and Ctrl-Click
Shift-clicking the amount button temporarily resets the amount to 0% which effectively disables the transformation. This is a quick way to compare before and after versions of the transformation.

Ctrl-clicking the amount button temporarily resets the amount to 100% which enables the transformation at full strength over the entire input image, even if it has a mask. For zero-centered amount controls, both Shift- and Ctrl-clicking reset the amount to 0% which is the center of the amount scale.

Observation
When the amount is temporarily reset using shift-click, in a compute-intensive transformation ( for example, Precise Gaussian blur ), the computation delay can be significant to the user, perhaps several seconds, even when displaying the effect only in the preview window of the dialog box. It seems that the image transformation is processed again.

Desired behavior
When reverting to the input image and when going back to the effective transformation, the user interface delay should be un-perceivable.

A better way is to use Ctrl-D (or equivalently alternating the display between the input and output images using the tool bar) which switches between the input and output images and does not involve recalculation.

This is different from changing the Amount control, particularly in cases where transformations have more than one since it only resets one of them.