It is used by holding the free end of the string ( usually by inserting one finger in a slip knot ) and pulling at it so as to cause the spool to turn while it is hanging in mid-air , either winding or unwinding the string . It is played by holding the free end of the string ( usually by inserting one finger in a slip knot ) and pulling at it so as to cause the spool to turn whilst suspended in mid-air , either taking up or releasing the string .