The concept is a system that puts out equal or greater energy than put in, which is impossible due to inefficiencies (friction, heat, energy losses etc...)
SWET's scheme is a variation.
"...perpetual motion machine ? how does such a machine run ? ..."
"It is easier to fool someone, than to convince them they have been fooled."
For those that don't get it, it means that people need to be more careful, and do good due diligence.