Complex sentence: On the UX (EZM 2B), the case back contains a large movable piston with an o-ring seal, allowing the liquid inside the watch case to expand and contract to adjust internal fluid volume and equalize with outside pressure.
Split into simpler sentences: On the UX (EZM 2B), the case back has a large movable piston with an o-ring seal. The piston allows the fluid inside the watch case to expand and contract to adjust internal fluid volume and equalize with outside pressure.