To be honest, I didn't even know that this had a name until 5min ago!
It refers to when you buy a return ticket but do not use the second leg of the journey.
I was planning to do this to save money, as a same day return flight to Amsterdam is more expensive than two return flights 
So I want to fly
London-Amsterdam on 15 Sept, (return at a later date)
and
Amsterdam-London on 15 Sept (return at a later date)
Does anyone has any experience, or knows if KLM has any clause against it?
Thanks!