When the ball becomes older and scuffed up and you have kept one side of it shiny, it will move in opposite direction without any major changes in the seam position.
For example, with the rough side on the left and shiny side on the right and seam position directed towards second slip, the new ball will move away from the batsman whereas the old ball will move in causing reverse swing.
Simply put, in conventional swing the ball moves in direction of the rough side, whereas during reverse it moves in direction of the shiny side. There is only one condition, the ball has to bowled above 80mph.
Pakistani pacer Sarfaraz Nawaz discovered this art in 1970 and passed it on to Imran Khan who passed it to Wasim Akram and Waqar Younis who shared the knowledge with rest of the world.