This macro auto selects between 2 inputs (image / branches). It selects the first input if you are working in the gui or the second input if you are rendering (or commmand line).
Use for an interactive proxy or something that you want to skip while working interactively that takes a lot of crunching. ie: heat distortion, motion blur, a baked out still matte painting from stitched tiles in motion.
(I was tired of waiting for shake to read & process 12 2k sequences that made up a full motion, tiled bg / matte painting. So baked out one 10x2k still, that i could then workout the tracking / camera move on. Also used for some large overall heat distortions that covered the entire comp.)