Coastering is replacing the reed valves on each cylinder head with a 'coaster'. This is a metal plate that replaces the valve and plugs the port (same as a marble would do). Some people prefer the look and even polish their coasters.
The reed system has several negative impacts to performance and ride-ability, the worst of which is excessive backfiring during deceleration. Other impacts of this design are Exhaust Pipe bluing, excessive exhaust heat, and engine heat.
For more information regarding Coastering and how to reduce popping and backfiring, click HERE