A tuned pipe is generally longer than a conventional muffler and is designed to resonate at a specific frequency (a specific RPM).

The result of this resonance is a suction at the exhaust outlet of the engine which helps to rapidly scavenge the exhaust from the engine and haul in a new fuel/air mixture. This allows more power to be generated.

The downside is that maximum power at only one RPM, a lowering of fuel economy, an increase in "fussiness", and an increase in engine stress.

The upside is a really cool sound, a really fast plane, and the admiration of all who watch you fly!