When there is an axial movement, the in-line bellows will start compressing and the balancing bellows will extend and vice versa. Both bellows will be moving.
![In line pressure balanced expansion joint diagram](https://devptp.usbellows.com/wp-content/uploads/1990/07/Pict-300x144.jpg)
When there is an axial movement, the in-line bellows will start compressing and the balancing bellows will extend and vice versa. Both bellows will be moving.