Caution: this bicycle accessory may instantly tranform your 6-year-old child into a pint-sized Hell's Angel. Turbospoke is an attachment for the rear chain stay. Rember the old "playing card in your spokes" trick for making basdass noise on your badass bike? This is like that, only badasser. Link (thanks Marc)
Update: BB reader Matt Dowling says, "On an episode of the TV show Braniac in the UK, a kid wanted to know if he was faster or slower with the turbospoke attached to his bike. So they set up a little circuit for him to pedal around with the thing on/off. Realisticly it should slow him down, but the result was that he went faster with the thing on, and what they figured is that it was psychological in speeding him up (with the "vrooom" and all)."
