A Trapped Cord Strangely Jumps From Its Position On A Board

How can a cord, tied to a stiff board, jump from one position to another?The audience is shown how this could be done, but then the impossible happens.
A cord, tied to a stiff board, jumps from one end to the other.
The magician then shows the audience how the trick is done.
But as a surprise ending, the magician causes the cord to jump to the middle of the board, much to every-one's surprise.