This  Booster fits Denis Wick Cornet mouthpieces. This description from the Denis Wick product website explains what it does. "A booster converts a Classic mouthpiece to a HeavyTop by adding extra mass which fits snugly around the existing mouthpiece. The booster’s extra weight helps to transfer more energy through the horn and to the bell, resulting in a more powerful sound, more secure high register playing and more focused tone. Extra mass gives greater projection and also darkens the sound. The tone develops a deeper ‘core’ and takes on a darker quality, giving a greater depth even at extreme volumes and ranges without the sound breaking up."