What is 'Snicko' and how does it work?

[BBC]Sweden's fourth goal against Tunisia at the World Cup on Sunday night was controversially awarded following the use of Snickometer technology.Mattias Svanberg scored just 18 seconds after coming on as a substitute in the second half by converting a Yasin Ayari free-kick during the 5-1 win.Initially, Svanberg had been declared offside at the time the free-kick was delivered.Following protests from Sweden's coaching staff and players and after a check by the Video Assistant Referee (VAR) team