Kurt Vonnegut gives a explaination of this term in Hocus Pocus.

The author taught me a new term, which was "Finale Rack." This was apparently from the vocabulary of pyrotechnicians, specialists in loud and bright but otherwise harmless nighttime explosions for climaxes of patriotic holidays. A Finale Rack was a piece of milled lumber maybe 3 metres long and 20 centimetres wide and 5 centimetres thick with all sorts of mortars and rocket launchers nailed to it, linked in series by a single fuse.

When it seemed that a fireworks show was over, that was when the Master Pyrotechnician lit the fuse of the Finale Rack.