From Berp & Company:
The buzz extension and resistance piece, better known as The Berp, helps you develop proper breath support
by blowing into resistance, letting you buzz your mouthpiece while you hold your instrument in the regular playing position.
The Berp also lets you press your instrument’s valves or move the slide to match the pitches you’re buzzing, so you gain
ear-training benefits through reinforcing the connection between buzzing the mouthpiece and playing.