I imagine this feature like this:
your clips are playing, parameters animating,
push a button which brings you into 'Preview' mode.
your composition and clips are still playing, parameters animating, but anything you do in the gui does not affect your output now.
You can launch clips, change parameters, you see what you do in a preview monitor, but your output is still the same as before.
When you have finished adjusting stuff, set a fade time for the transition between the states, push a button to transition to this state.
If you don't like what you did, you would also have an option that would get you right back to the live state without changing anything.
Obviously this would be quite heavy on your GPU, your composition would be rendered twice in cases.