The purpose of a misfiring system is to hold boost even while shifting and such by keeping exhaust gases flowing. This way you'll never have to wait for your turbo to spool up. The only real applications I remember it being used in is rally but I don't even think it's used much there. I had a good link to a site about it, try checking google. It had stats about it..
Your average stock car off a dealership would not go 50km with a system like this before the exhaust system was completely destroyed...
I'm not 100% sure on the way it works but the general concept is to keep exhaust gases flowing at all times thus keeping your turbo spinning. I *think* it activates when you push in the clutch or let off the gas and what it does is keeps the pistons and stuff going, but the exhaust and intakes valves are momentarily open at the same time so you've got fuel travelling into the exhaust manifold where it detonates instead of inside the cylinder. This is the source of the popping noise and flames coming out the exhaust. I'm not sure if that's 100% correct.. if it's not I'm sure someone will correct me on it. I do know that it did involve detonations in the exhaust though and I think that was the reasoning. The detonations would keep the turbo spinning.
Ok, here's the link to the website all about it:
http://www.rallycars.com/Cars/bangbang.html
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When the driver lifts his foot from the gas pedal the ignition timing is altered with sometimes 40° or more of delay (retard) and the intake air and fuel supply mixture is made richer. The inlet butterfly is kept slightly open or an air injector is used to maintain air supply to the engine. This results in air/fuel mixture that keeps getting in the combustion chambers when the driver no longer accelerates. The ignition being delayed, the air/fuel mixture reaches the exhaust tubes mostly unburned. When the spark plug fires, the exhaust valve is starting to open due to the ignition delay mentioned above. Additionally, the exhaust temperature being extremely high, the unburned fuel explodes at the contact of the exhaust tubes. Luckily the turbo sits right there and the explosion keeps it turning (otherwise it would slow down since its intake, the exhaust gases, is cut-off). The effect is vastly lower response times with some downsides.
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It works by altering the ignition timing.