[FFmpeg-user] decimate=ppsrc question

[Mark Filipak (ffmpeg)]

Can anyone kindly give me a usage example of this:
ffmeg -i SOURCE -vf decimate=ppsrc TARGET

I'm totally dumbfounded by this:
"ppsrc
     Mark main input as a pre-processed input and activate clean source input stream. This allows 
the input to be pre-processed with various filters to help the metrics calculation while keeping the 
frame selection lossless. When set to 1, the first stream is for the pre-processed input, and the 
second stream is the clean source from where the kept frames are chosen. Default is 0."

Specifically:
"the first stream is for the pre-processed input": What is a pre-processed input? How processed?
"the second stream": What 2nd stream? Are there 2 inputs? If so, does that mean I need to use a 
filter complex? Or are there 2 passes? If so, how do I form the command line?
"clean source": What is a clean source? How is it made clean (whatever that means)?

Thanks for your help.