This app works by measuring the air pressure in the device while you squeeze the screen. If the air pressure in the device changes significantly it means the airtight seal is probably intact. If the air pressure in the device does not change it means the airtight seal is probably broken.
REMEMBER! 1. Squeezing or applying pressure to your device may cause damage. 2. This app does not actually determine if your device is water resistant. It is only an indicator. Always refer to the manufacturer's guidance. 3. This app has only been tested on Sony Xperia Z3 and Z5 devices.