Interpolate/Extrapolate plug-in does a simple linear interpolation between two images. However, it does not constrain the blending factor to be in [0, 1]. By using values outside that range, you can perform more interesting extrapolations among images.
For examples on how to use this sort of plug-in, check the Grafica Obscura pages. As an interesting example, below is shown a color image and its grayscale version. Extrapolating `away' from the grayscale image leads to an image with increased saturation.