Just did another quick test. I called up a G chord then I clicked 'set-chord-hold' and this changed the G chord to a hold. I then clicked 'Play over shot, hold or rest' and chose Bass and drums. I then saw that my G chord had changed to B diminished. Other odd things happend when I called up another chord. I then made that one a hold and then I clicked 'Piano' and this time it move the bar with a blue fill marker and if I clicked piano again it turned it to a green marker.