On rotary valve engines there is a sudden vacuum drop on the carb side. That drop is sucked back into the boost bottle that is mounted facing nozzle down, and when the valve opens that fuel is sucked out into the intake. They work wonders on rotary valve engines. They can add up to 10% more low end torque.
But on reed valve engines they just look cool. Unless Everyone knows they are just glitter... Then you just look stupid.
