When I go home from my buddy's house last night (aprox 30 mile ride) and shut my scooter off, a large-ish pretty blue flame shot out of its butt! Sonically, it only emitted a soft pfft but it was visually striking!

You can achieve that cool blue flame yourself, with a match,,

When you shut the engine off it stops the ignition - but it does not stop the air flow through the carb and the mixing of fuel.  The air and fuel that goes through the engine and into the exhaust unburned.  If the pipe is warm enough to heat the fuel to a temperature that it will ignite - then it burns in a slow growl as it is not compressed as it would be in the cylinder.

I think it is kind of cool to hear.....and I don't consider harmful or a design defect.

Dave wrote on 06/25/12 at 09:20:11:

Great explaination of what a poof is.

And the flip side of that is the Bang...
Lean burning causes missing, unignited fuel collects in the muffler, on shutdown, fresh air/fuel enters the muffler, when this hits the hot muffler, it ignites as well as the unburnt fuel causing a much louder explosion.  tune for the poof for efficiency.