It isn't.It is a variation on this experiment
http://viscog.beckman.uiuc.edu/grafs/demos/15.html
Daniel Simons of the University of Illinois at Urbana-Champaign and Christopher Chabris of Harvard University won the IgNobel for Psychology in 2004 for the research
http://www.improb.com/ig/ig-pastwinners.html#ig2004